Do you know if it has hydraulic lifters?

850s have hydraulic lifters and have almost no valve train noise.

Solid lifters or tappet buckets make some noise when cold and quiet up a bit when warm.

The B20 in my 544 has solid lifters, it is not really noisy, but does have a mechanical sound to it - it does not make more noise when accelerating as other sounds mask valve train noise.

If your car has solid lifters (I have driven one on a test drive and I think that it does) , have they ever been adjusted according to the schedule in your owner's manual? Many people fail to do this as the engine will run even if they are out of adjustment.

I would call some Volvo indies and dealers to find out if any of their techs are familiar with your engine (many are not). A mechanic who hsa worked on 16v engines probably can tell you if the noise is correct in a minute.

BTW - do a search for Volvo shops and dealers on the internet - I was surprised how many indie shops were listed for my state, MA.