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Morning!

Over the past few months I've noticed that when the AC is on and I'm driving at low speed (approaching a junction/roundabout etc.) there is a loud whirring noise coming from the engine bay. It doesn't always happen though. I can be driving around with the AC on and no noise and it's blowing nice and cold, but sometimes it produces this noise. It sounds like something is clearly wrong so when I hear it I generally turn the AC off and it stops.

My car has the manual heater control dials with the "AC" button (rather than climatronic).

I have also noticed that the v-belt/aux belt looked worn with some wear marks on the back surface. The tensioner pulley also had some deposits on it's surface, which looked like little bits of belt material. At the weekend I replaced the v-belt and cleaned the deposits off the tensioner pulley by simply scraping them off while rotating it.

I'm thinking that these things are related.

I also feel that when the AC is on the engine revs get pulled down between gear changes more than they should. It's only really noticeable at low speeds but I can detect it. Maybe this is typical but I've never noticed this on my Octavia.

VW performed a "service" on the car last week and I asked them to investigate. They spotted the worn belt and deposits on the pulley and recommended replacement of both (£230). They also claim that there is a fault with the compressor pulley and that the entire unit should be replaced (£1060).

Has anyone else been through this? Any advise basically?

What are my DIY options? Can I really not replace just the pulley? I have found a new genuine DENSO compressor for £300 from carpartsforless. The original unit is DENSO and I have the part number to cross reference. Can I replace the compressor myself and then take it to a garage for the recharge and oil? I've never worked on AC before...

Any help would be great!

P.S. No error codes. It's the CGGB engine (1.4 16v petrol), RHD, manual, 2011 model.

This is not good news at all.
That new Denso compressor, I'd think that if you investigate further, the one fitted to that car is a 140cm pumping capacity and the one that you have identified is a 120cm pumping capacity - which is smaller, why they are only offering that one beats me and is typical of that company/group! I'd think that it would do its job in UK temperatures, but not the same as what is currently fitted - I only discovered that when considering replacing the compressor on my daughter's late 2009 Ibiza, though in my case it was a hasty incorrect bit of diagnosis as I thought that the compressor was leaking at it body gasket - I was wrong!
You should not try to do much work on that area yourself as that system contains an F-Gas which needs to be disposed of in the correct manner. Maybe get a mobile AC person to replace that compressor, but make sure that you get the correct size of replacement and not just the same make and model - these compressors tend to come in different pumping capacities in the same series with variations on the body casting to allow for easy bolting on to different engine types. You will also need to buy a new drier assembly and some seals as the radiator needs to come out to replace the drier - stupid but true, the new compressor will come filled with PAG oil and you need to weigh out what oil you can drain out of the old compressor and remove enough oil from the new one to reduce its oil capacity at fitting time to being the same as the old one was.

The later versions of these cars were ll fitted with Sanden compressors and they seem to have been causing issues due to swarf being left in them when initially built up by Sanden - not good!

Good luck, but I think that you should avoid VAG workshops for that job and get a mobile AC person who lives by his good name, to sort that out - and he should be more able than we are to source the correct size of compressor.

Edit:- I'd think that it is the internals of that AC compressor that are failing/seizing and causing that pulley, with its shear coupling, to get damaged and until it is properly sheared, cause extra load on the pulley/belt and so engine revs at idle.