Hi 2002 V70 petrol .I have a underbonnet noise of which im not sure where its coming from.The sepentine belt tentioner looked worn,it was spinning a little unevenly ,so thinking this was the problem ,i replaced it and the belt,but still the noise is there .its like a slight rumbeling noise,So i took the belt off started the engine the noise has gone!!!! Im finding it hard to tell which of the units is making the noise,any ideas,90k miles.

 

That is a common problem to try to diagnose. You might be able to figure out which one by using a stethoscope. Or, you can put a solid object against your ear and listen that way, touch each rotating accessory. don't get hurt.