A very common cause of vibration is the right most engine mount, just behind the crank pulley. When it fails the engine rests on the subframe. Check it out.

Very few people are satisfied with the solid torque mount on 1999 and newer Volvos.
When I installed it on my 2001 S40 2.4T I thought I killed one of the cylinders by mistake. The front bumper was shaking loose and the steering wheel numbed my fingers.
I installed the original and everything went back to normal.
I thought it was only the S60, so I installed it on my 99 V70 XC and the same thing happened.
My 1996 and 1998 don't mind the solid mount.

You shouldn't feel ANYthing at idle. Zip, nada. Check your vacuum lines/elbows for cracks and clean the throttle body and idle control valve.

Klaus
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(V♂LV♂s 1975 164, 1995 854T, 1998 V70R)

It really depends upon where your vibrations are coming from. You could probably benifit from a new set of engine mounts if you have not done that already. I have also had a couple of the S/V70's that set up a nasty vibration from the front coil springs but only when in gear and at a stop. Usually this phenomenom can be dealt with by insulating the front coil springs with a piece of rubber hose. Believe it or not it was an actual factory fix back in the day. They had us installing about 12-18 inches of rubber hose on the lower most coil. Just slit the hose install it on the spring and hold it in place with hose clamps or zip ties. Every once in a while you get a car that you need to replace the struts in it to fix that noise. The truly unfortunate folks end up getting new engine mounts, the rubber hose treatment and a new set of struts and spring seats to take care of all of this B.S. Depending upon the mileage and nature of your rattle, I would probably start with the engine mounts as they are the more common failure. You have already figured out that stuffing in a urethane bushing only makes the vibrations worse. I like the urethane bushings but they can worsen a bad situation.

Good luck
Mark