On my 1987 740, I'm noticing a "hum", intermittently from what I think
is a vacuum device. If you look at the cable spool attached to the
throttle body, a cable on the end goes toward the firewall, through a
diaphragm looking, black rubber 'thingy', and into the firewall. With
your hand you can compress this device, as it is soft rubber.

With the engine idling, every few seconds it hums then stops then hums
then stops.  Can this be repaired, replaced? What is it?

Thanks!