Noisy E-Unit

I have a PW # 2338 Milwaukie Road engine where the e-unit makes a lot of noise most of the time and there are times where it runs quite.  I have cleaned all the contacts and the barrel on the e-unit with contact cleaner, but still no change. Any suggestions on what else I can do?  What causes (or wears out) the mechanical e-unit to make a lot of noise where others runs quite?

Thanks

Bob D