[R-390] Microphonic Parts?

[Barry]

I know that tubes can be microphonic.  Is it likely for passive components to be microphonic as well?

I have an old Marconi LCR bridge that I'm trying to get working correctly again (it doesn't want to stay NULLed in R mode).  Whilst poking around at the input of the first amplifier (an EF86) with a small plastic stick and looking for bad grounds or solder joints, I noticed that a very light tapping on the components connected to the control grid causes quite a lot of noise at the detector (as observed on the scope).

The components that seem to cause the most noise are a 1M resistor and a 470pF cap.  The cap is connected directly to the grid and the resistor is connected on the other end of the cap to ground.

I've loosened and retightened the ground points at the tube base and gently pushed on the solder joints but nothing seems to produce this noise like the tapping does.  Tapping the tube itself produces a small bit of noise, but not nearly as much as these components.

Is this "normal"?  I tend not to think so, but the control grid of the first amp is a pretty sensitive place to go knocking around on so I'm not sure if this might just be expected behavior.

Not directly related to an R390 but I know you guys are a great source of help for things like this.

Thanks,
Barry - N4BUQ