'97 850 GLT
I have a strong vibration at idle coming from the steering pump. Or under/around it? I have no idea where the source is. Once I rev it, it naturally smooths out. All bolts and everything are tight around there to.

 

How's that motor mount up front?

 

Motor mount under the balancer, good choice, maybe a harmonic dampeners (the crank pulley) rubber is allowing some wobble and straightens up with more RPMs ?? Have you eye balled the AC compressor and clutch? Take the belt off and feel all the pulleys as they turn for anything out of normal.

When you first start it up, before you race it any and it's doing it's vibration thing pull the PS cap and see if the fluid has any bubbles or looks whipped. There shouldn't be anything the PS pump itself can do other than a loose or bent pulley and I'm guessing you've looked all the pulleys that serp belt touches over good.

Vibrations and noises are hard to do from another state !