Noisy Central Heating Pump

I'm back for hopefully some more sound advice...

One of my radiator valves started to leak. I'd put off fitting a towel radiator in the bathroom because I didn't want to drain and refill the system, but decided it was time to tackle both, and treat the system to a clean and some new inhibitor.

So I drained the system, fitted the new valve and towel radiator, refilled the system with cleaner additive. All radiators were bled and heating nicely. I noticed that the pump was humming louder than usual. I put it down to air in the pump, and knowing I'd be draining and refilling in a week I left it.

I've now drained the system again and refilled with inhibitor additive. All radiators bled as well as pump (as best as I can) and everything is heating. I've noticed now that the pump is even noisier still (https://www.youtube.com/mWGbZr5_Yng).
I've tried repeated bleeding of the pump with no difference. I've scoured the loft for more bleed valves but there are none. Is it just trapped air or is the pump on its way out? Should I try cracking off the valve above it to see if any air will escape out of the top?

Here's a couple of pics of the setup.



Many Thanks

James

So you have undone the silver screw on pump to bleed air?

Tony

Hi Tony

Yep bled it through the silver screw. I've bled it again tonight and a bit more air came out. It's really difficult to tell when there is no more air.

To be fair, and I have an old system including some microbore pipework, to get all of the air out can take weeks. When last drained, my pump was making noise for a good few weeks despite bleeds daily!!