I have a 2001 S40. Sometimes I notice that when I'm stopped with the car in drive (with my foot on the brake), the whole car seems to vibrate. When I put the car in neutral (again, foot on the brake), the vibration completely goes away.

Could this be the engine mounts going bad? It doesn't happen all the time, mostly when the engine is cold.

 

Definitely engine mounts. There are 4 mounts on the 2001. I'd replace the 2 underneath the engine first to see if that resolves the vibration. This worked for my 2001. It's fairly easy to replace these too. After removing the plastic shield under the engine, remove the bolts between the mount and the engine then remove the long metal piece to which the mounts are attached. Now the mounts are easily replaced on the long metal piece.

The main engine mount on the top pass side of the engine is fairly expensive as you have to buy both the aluminum arm on the engine and the mount itself due to a Volvo redesign of the mount. This one does wear out eventually though.

The mount on the driver's side above the trans is the last one I'd replace. On mine it was still in perfect condition when I replaced the other 3 mounts.

Let us know how you fix it?