TD5 tick over

Hi all,
Summers arrived here with temps up towards the 30's so my air-con is being used again. I've noticed that when idling without the heater on or in econ mode she ticks over at +/-750rpm but when the air-con is engaged she sounds a little rough as if the revs have dropped and there is an occasional knocking sound from the engine. The rev counter doesn't move though. Any ideas? It only happens when idling.

thats the air con compressor.

it is soleniod controlled on the pulley, so the pulley still spins but dont always turn the compressor. when your air con kicks in you get the knock followed by lower engine speed 'cos of the load of the compressor.

get yer head under the bonnet, watch the pulley while your misses switches the air con on and off...

I know what the sound of the compressor kicking in makes, but this sound is almost like the sound the engine makes when the anti-knock sensors are kicking in cos the idle speed has dropped too much.

Bi-mass fly wheel breaking up ?