Wondering if anyone else has this problem if so what’s the cause & fix… I have a customer car that is making a strange resonance hum from the heater plenum area but only when the temp control is moved to hot. It’s RPM sensitive too. Usually from ~1800 RPM up. The sound can best be described as a wooowoooowooo at a fairly fast cyclic rate. Another description is think of strumming your fingers across a heater grate/diffuser in your house and you get that harmonic hum for a brief second.
It’s an original unmolested automatic 68, 442 with factory A/C. The sound is coming from the metal grate area where the plastic plenum attaches directly above the trans tunnel? It does it with and without load on the engine? This one’s got me perplexed!

 

There was a factory tech bulletin about that sometime around mid-1973. It was called heater "moan" but was addressed more to 1971-later cars. The quick fix was a kit that put a brass tee in the lower radiator hose and routed the heater return hose to it. The permanent fix came in 1974 when they routed the heater return hose to the radiator cold side tank instead of the water pump.

This may not be the cause of your customer's problem, but it sure sounds like what that tech bulletin described.

 

Never thought of coolant cavitation or harmonics? Owner says it never did this until the trans was rebuilt and a new converter was installed, coincidence? Its definitely not coming from the trans because I had it on the lift with a stethoscope. It’s definitely coming from the heater plenum and not the heater core though the two are close enough. It’s has TCI 2800 stall converter with a stout shift kit and other internal upgrades which I installed 4 years ago. This is one of the cars few deviations from factory parts that I can see other than normal sundry items of course.
The heater valves been replaced a couple of times due to the usual leakage from sitting. She has 62K original documented miles of which he put most of that on himself. He’s the second owner. I’ve driven it and it is the strangest thing I’ve ever encountered in a while. This has been a very elusive problem, one that I’ve never heard before in my 40 years of trouble shooting. It will do it in park as well as under load.
It would make some sense that it has something to do with coolant because the heater valve would be signaled open when the temp control is moved to Hot? Or is that counter-intuitive to the tech bulletin? I’ll try disconnecting the vac signal to the heater valve and see if just moving the blend door produces the same moan, cuz I'm sure scratchen my head on this one.
Thanks for the info. At least it makes some sense and gives me a direction.