Belt noise and valve train noise are two different things and I guess you should isolate the noises in your driveway to make sure it's not piston slap. In the meantime I would get the poly bushings that ipd has; they allign all your belts correctly which means less friction on the engine, less wear on acessories and it quiets the noises that the belts make especially when new belts and a new t-belt and tensioner are added to the equation. You will see/hear good results with these at only like 40-50 bucks. I put them on my 82 brick with 200k miles (original bushings) and all the offensive noises went away except for some faint valve "tapping".

I think some valve train noise is expected, but a "valve job" takes care of that in short order. This entails new hushers, new guides, maybe a shim or two, seats, etc etc. I'm sure many here will chime in that they have permanent valve train noises on their volvos from the sewing machine noises to all out tapping. Your 900 is 16 years old, it's gonna make some noises - which may only be an indication that some components need to be refreshed.

Also be sure your exhaust manifold gasket is OK. A tiny pin hole can sound like a machine gun at speed. It's like 8 bucks and is easier than you'd think to replace.