Sound can be heard in this video: https://youtu.be/nZetfaCiQHI" rel="nofollow - link .

Any ideas as to what may be causing this noise?

Sorta maybe sounds like it could be a ball bearing that's not in the best shape.

I'd spin the circulating pump by hand with the belts off and see if it "ticks" a little.

There's always the chance of an exhaust manifold gasket leak which can sound like a variety of things while attempting to  drive you out of your mind 

Exhaust manifold gasket leak is definitely a possibility, given the customisation that went into fitting my manifolds to the heads. Is there a better way to check for a gasket leak other than looking for some exhaust smoke leaking out when the engine is running?

I suppose you have twice as many chances for an exhaust gasket leak with your spacers/adapters but after listening to the noise till it drove me half out of my mind, I'd probably look at the raw water pump first 

The remaining part of my mind says small exhaust leaks are tough. Have you checked that all of your mounting bolts are tight?

If the pulley is tight on the shaft and the key is a good fit in the keyway and the impeller is a good fit on the key, then that movement is the flexible impeller vanes.

I don't think you ever mentioned whether the noise went away when you ran it for a quick check with the belts removed

This will have to wait for a while. I've already pulled the transmission from the boat, and running the motor without the exhaust hoses connected to the risers is so loud there's little hope of listening carefully for presence/absence of the ticking noise. I'll get back to this when I have my transmission back in the boat.

The front of the timing cover looks pretty nasty to me. So:

1. Does the corrosion on the front of the timing cover and back of the water pump cover plate suggest that the water pump has been leaking? Any way to check/confirm?

2. The circulation pump spins reasonably easily by hand when removed from the block, and there is no obvious free play in the bearing. But there is definitely a slight whining noise when the pump is rotated clockwise (i.e. the opposite direction of rotation to which it will normally be spinning on my reverse rotation engine). Does that noise suggest the bearing is bad, and potentially the source of the yet to be identified ticking noise? Any further tests/checks I can do on the pump bearing to confirm?

3. Anything else that looks odd or in need of attention in the water pump from the pictures?

As always, thanks a lot for any help!

Since you have a closed cooling system, presumably with anti freeze in it, it would probably be a good idea to replace it since it's pretty ugly looking. 

Looks like a lot of years without anti freeze to me

The typical PCM pump sold these days is bi directional and has a much beefier cast bronze impeller and a stainless steel backing plate.

Here's a picture of a well aged marine pump with the backing plate off.

If what you have is working and you don't have cooling issues, it would be your choice of what to do.

Well, now that you've mentioned https://www.correctcraftfan.com/forum/forum_posts.asp?TID=47357&PN=3&title=pcm-351w-fresh-water-cooling-fwc" rel="nofollow - it ...

I have of course been having cooling issues. Have been monitoring the situation carefully, and after doing more research on some of the car forums I concluded that my engine temps are probably on the upper limit of acceptable. But they have certainly been hotter than I was expecting with a 170 degree thermostat. Maybe this water pump is the root cause of that. I'll definitely be replacing.

This one from Skidim is obviously the right one: https://skidim.com/circ-pump-ford-bi-rotational.html" rel="nofollow - link . Any other suppliers worth checking for the right pump at a good price? Nautiqueparts.com seems more expensive. I have a local reseller who can usually supply me with parts from Summit Racing at a decent price. This Sierra Marine pump looks like it is the same pump as the one Skidim sells, but at a slightly lower price: https://www.summitracing.com/parts/rra-18-3584-1" rel="nofollow - link . Any reason not to go with the Sierra Marine pump from Summit?

It's probably the same pump that SkiDim is selling

I knew you had cooling issues in the past, didn't know if they had all been resolved though. Like you said, maybe this will help.

Use your favorite sealer on the gaskets. Put it on heavy where the little pan corner gaskets butt up to the block and where they meet the neoprene seal.

It's not as easy as doing it on an engine stand with the pan off, but it's not terrible either 

Questions:

1. Is the timing cover still serviceable, or should I replace?

2. Is a helicoil and mounting bolt in the other non-threaded hole a good idea to help avoid a leak, or is that just unnecessary effort?

The helicoil............not required but couldn't hurt.

https://www.ebay.com/itm/352101911892?hash=item51fae8cd54:g:VE8AAOSwd4tTrc8f

So can anyione advise me as to whether:

1. That is indeed an existing sleeve on the snout of the balancer, and

2. If it is, whether it should be removed before installing the new sleeve, or whether the new sleeve should be fitted over the old one.

Thanks as always for any guidance!

The original diameter is about 1.875 inches and with a National brand speedi sleeve, it's about 1.900 inches.

I don't think it's a sleeve, but that's just me thinkin' 

If it is a sleeve, you'll have to take it off before putting a new one on.

The cam sprocket I reckon I'll be able to draw in by tightening down the cap screw that holds everything in place on the camshaft. I can't imagine that this will require so much force as to risk damaging the camshaft's internal threads. But it seems the crank sprocket will require either heat to expand it and allow it to slide all the way in, or some gentle tapping with a suitably sized piece of pipe to drive it all the way in.

Any suggestions as to the best route to follow?

You might need to clean up the key and the shaft a bit to let it go on easily

Click on the link for some good info 

Said key doesn't want to come out without a fight. The parts manual diagram suggests it is a semi-circular woodruff key, so if I hit down on one end with a punch or such like, the other end should pop up and out, right? Or is there a better way to get the key out?

if it's putting up too much of a fight, leave it alone so you don't end up mushrooming or otherwise damaging the key or the shaft. Sometimes they're a bear to get out and if you don't absolutely have to, just clean up the shaft and key as much as you can.

Keep in mind that you're working right over the open sump so any dropped stuff ends up in the oil pan 

After not planning to do so initially, I decided the timing cover really does need a new paint job. Took me a while to get it properly cleaned up, but I'm now ready to paint. Two questions:

The only thing in there is air or water leakage from the pump backing plate or the outlet ports where they meet the timing cover. That's probably why they weren't painted originally 

As far as RTV or gluing with something like Permatex 2 or something similar use whatever you feel more comfortable with.

I've been known to trim a gasket or 2 in that area to make it fit better. That gasket isn't really any fun at all