I'll tell you a funny one about an "intermittent noise".....
An old Minneapolis Moline farm tractor that I take care of for an old widow lady had an absolutely terrible grinding/growling noise at certain RPM's - sometimes low/sometimes high. Didn't matter what gear, or if moving or stopped, but was bad enough it was hard to sit in the seat. It rattled my brain. [assuming I have one]
While motoring the tractor[s] to and from the show grounds for the local tractor show the deceased's brother, who was helping, asked what was going on. I said that noise has always been there since I've been taking care of the tractors. He said when we get time we'll have to tear into the rearend to see whats going on.
This went on for several years. The tractors only got out of their shed once a year for the local tractor show. Minimal maintenance, I'd just check the oils, antifreeze, tires and charge the old 6 volt batteries and run them.
The brother was standing there listening to the horrible noise one day at the show and he put his foot on the drawbar and the noise stopped.
All the noise was - was the engine vibration resonating through the drawbar and being amplified like a speaker. I grabbed a small piece of wood and drove it in the drawbar cradle to take up all the slack and its been quiet ever-since.