As Gerald said it is difficult to diagnose an intermittent.....If you are handy, I would consider changing out the switch which gives you a new contact set and renewed connections at the switch and also the solenoid switch at the pump motor. Those items are relatively inexpensive and depending on position and access, not too hard to change. Both of those contain contacts that can oxidize, pit from arcing (especially the solenoid switch), and corrode. If that fixes your problem, then great. Be sure the batteries are disconnected that feed the pump solenoid switch and the charging system is off. If the battery connections have corrosion on them that needs to be fixed, that can be part of the problem if low voltage is what is reaching the pump. With those items ruled out it is likely that you may have a "dead spot" on the pump motor where depending on where the rotation stops, can be intermittent.
I am a little puzzled by the noise in the cabinet behind the switch. If an additional relay is in the circuit there, that is a contact set also and if it is available, I would consider changing that also. Basically you are changing out all the "switches" on the way to the motor. This is a bit of throwing parts at the problem but they are inexpensive parts that rule out issues and probably cost less than 1 hour of labor at a shop. It is a trade off cost wise but renewing some contacts on critical systems in older coaches is also a bit preventative.
Later Ed