If it always makes noise, even at idle, it should be easy to find out which part it is. Rather than a stethoscope, I have a long screwdriver that I touch to the suspected part and the plastic handle touching my ear. You could also use a piece of wood, like an old broom handle, anything hard that will transfer the vibration.
Check the idler pully too.
If the noise cannot be traced that way, take off the cover to the timing belt and check those part also. Just be careful, there are a lot of spinning parts to grab fingers, etc.

Klaus
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(V♂LV♂s 1995 854T, 1998 V70R)

Before you can go any further, you need to figure out what's actually making the noise. I highly suggest picking up a mechanic's stethoscope. It will allow you to properly isolate the source of the noise. Then go from there. Odds are it's either the alternator, or the tensioner. From personal experience, I'd bet alternator. But no point in throwing out guesses when all you need to be sure is a mechanic's stethoscope. No mechanic should be without one. And unlike medical ones, they are quite affordable.

Have you changed the Serp. belt lately? Should be every 60K miles. On the second change, the tensioner should also be changed. I suspect this, the belt itself or maybe even the water pump.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.