When I apply the brakes on a long downhill, I get a low frequency rumble from one side. I have the car up on stands, and have checked the wheel bearing (ok) the rotor is not loose, the pads look good, and the various rubber bushings all seem to be flexible but not loose. The only thing I can find wrong so far is that one of the front subframe mounts (drivers side) is snug while the passengers side is sloppy. The noise seems to be on the passenger side. Could this be it?
Can I drive the car until the new part comes in?

Some time ago our village mechanic suggested I try changing the brake pad clips. I've had them all winter and when I put them in along with new pins. the noise pretty much went away.

Note to self: in a car where everything is worn out, be careful not to overanalyze the problem. We all do that.