Well, I've been driving my 2001 S60 2.4 for some 6 months, got fixed everything that was broken including suspension works since it has sporty suspension that does not handle bad roads very well and very long (totally invested some 2000 euros). Anyway, lately, when there's finally a quiteness inside the car and nothing rattles, I'm hearing very weird noises coming from engine department - firstly, at 5th gear (manual) driving at 85-90 km/h with 2100-2300 rpm a sudden noisy vibration occurs (sounds lika a war jet plane take-off) and stops as I accelerate more and drive faster, or decrease acceleration and drive slower. Three different service stations have inspected my car incl. Volvo dealership and none of them had even a guess what the hell is that. Oh, yess, I have engine pillows changed, so it might not be a problem.
Secondly, driving slowly at high gears and low revs at 1000 - 1100 rpm and making a sudden acceleration, engine gives out a boiling sound - sounds like between a detonation on old cars and dead exhaust (which is in order). This sound showed up quiet recently, so I'm very concerned about it. I started to wonder - perhaps it's something with engine adjustments, ah?

I would highly appreciate any experiences or comments on this as well as your patience reading all this. Thanks!

I have seen it a couple of times before.SOme of them were a heatsheild vibrating on the cat conveter.I saw one that had a bad cat converter.

I know the noise you are talking about it is annoying as Hell.