Ask an expert

Toyota Vitz, 2002, import. Prior to being serviced noticed when ideling / low revs on cold morning or at nights noise that sounded like slipping belt. Car was then fully serviced, new main belt put in but noise much worse not just when cold but when warm every time ideling & at lights when aircon is on. Car labours more & seems to be lower the revs. Noise is sometimes noticeable in same situation with aircon off. Would it be another belt , water pump of something to do with the aircon?

It's likely to be something driven off the belt, especially if the noise is worse with a new belt, which presumably is now correctly tensioned and placing greater force on the defective component.
I would explain it to your mechanic exactly as you have done here, and he should be able to pinpoint the noisy component (ie; alternator, water pump, steering pump, a/c compressor etc).

I would sugest you take the car back to where you had the work done.

They will listen to the tensioners or pulleys using a screwdriver up to the ear (dont try this as you need to know what you are doing or listening to)

If the pulleys and tensioners sound normal, it might be just the case of bad tension so it might just need tightening up