Noisy engine, any clues?

I'm after a bit of help identifying the cause of the rough sound I have in the engine of my 1991 Cruising Cabin with the 2.8 non-turbo diesel engine. It's most noticeable at low revs and when driving around town. I don't think I can hear it if I rev the engine while stationary nor when cruising at say 50mph+ but it is there at low revs while stationary.

I have just had the water pump changed in the hope it was that and some more fresh oil plus some thickener made no change. I recently changed the autobox oil as well, made no difference.

Video below with a recording of the sound.

Thanks in advance.

Update, I have found a loose tensioner pulley - aircon, I'll replace that and see how the noise is after that. Fingers crossed.

Further update: it wasn't the idler pulley, but removing the belts from the alternator cleared the noise, so hopefully, a new alternator will solve the issue.