TDi 2.0 high idle & noise

On Friday I noticed my 2.0 Tdi (148k miles) had a very noticeable squeaking noise coming from the engine. I think it sounds like the auxiliary belt, or possibly a tensioner for the same. Probably time to replace it, I thought.

However, on Saturday the car then started to idle at slightly higher rpm than normal - about 1000rpm instead of about 800rpm. It starts fine, then increases to about 900rpm after a few seconds. Applying the throttle for a few seconds takes it up to 1000rpm, and it stays there when idling until it's restarted. Driving performance does not seem to be affected. Along with this, it is much noisier when idling at 1000rpm. It sounds much more like a tractor than normal, or perhaps a bit like a 6 cylinder diesel!

I've read that higher idling rpm can be due to a faulty exhaust pressure sensor, but not sure how that would produce the extra noise. I'm going to connect VCDS as soon as I can, but in the meantime, any suggestions? Anyone experienced anything similar?

Thanks!