droldsmorland

Like the title says...Strange resonance or harmonic sound from the heater. Sort of a woowoowoo at or above 2K rpms.
Car is a 68, 442, Automatic with factory A/C. Restored to 98% original down to the bolts n hose clamps.
Only happens when the temp control is move to hot with one of the air flow selections selected to one of the choices heat, def, vent, ac etc.... it has to be on one of the flow selections not off. Noise goes away when the flow selector is moved to off regardless of where the position of temp selector is. Which is concurrent with how the vac system works. The blower motor speed has no effect. No noise when temp selector is moved to cold. Ive been trying to solve this on and off for years with this particular car. I have had other 67-68-69 and 70s with A/C without this issue. If memory serves me it started to happen after I removed and played around with the plenum under the dash to restore it. But I think that's the same time I replaced the control valve...did it in early 90s. I've sense purchased another (third since original) control valve as they all leak from sitting. I've narrowed it down to the heater core control valve on the intake. The ID of the replacements from 4 seasons is different from the original. Different in architecture (has a step fit tube) and in diameter (smaller). The original did not produce this noise. Ive tried drilling it out to open it up. I went as far as possible without breaking through the walls. Still produces the noise. I pinch off the hose move the control to hot...no noise. So I know I found it. I remember reading a tech bulletin somewhere that some cars back in this era had this same type of dealer complaint and the solution was the replace the lower rad hose or something to that nature. The cause was a harmonic being induced by a hose...I think.


Questions:
1. Anyone see the same tech bulletin? Do you have a copy? (dated late 60s early 70s GM Oldsmobile)
2. Anyone else experiencing this on a 68-69 442 with A/C?
3. Any other sources for these control valves?
4. Need a different set of eyes and some brain juice here.... out of ideas, hence reaching out to the vast knowledge base here. I cant be the only one with this problem?
Thought of simply installing a restrictor orifice nipple and deleting the valve but this is almost a "points" show car(WIP). Also thought of installing a restrictor in the valve. but that seems counter productive as the original inlet ID was larger.
I have pics of the valves side by side for compassion. Ill post later. TX all.

MDchanic

I've never heard of this, but it sounds like good troubleshooting.

I'd recommend keeping your eyes open for a NOS valve (they do pop up on rare occasion) and installing that.

If it's a lower hose issue, then be sure that your lower hose has the spring inside it, as slight changes in diameter could definitely affect harmonics.

I've got no other suggestions, sorry.

- Eric

Fun71

My car had a similar water rushing sound when I removed the heater control valve and used a straight hose nipple, so I agree that you are getting this noise due to the water flow through the heater core.

You could try different size restrictors in the inlet heater hose to see if that makes any difference. I'm imagining a Delrin rod with a hole drilled through the center, inserted snugly into the heater hose, and a hose clamp around the hose/insert to keep it in place. You could try with one size, drill it larger, try again, repeat. I remember Chris Witt saying the non-AC cars use a hose nipple with a 1/4" hole (restriction) so maybe start with that size.

edzolz

I had the same thing happen to a 67 442 once. It started when the harmonic balancer ring came off. I never really knew what was causing the weird noise. Sounded like a moan at a certain rpm range. Later on the water pump gave way and it never moaned again after replacing the pump.

droldsmorland

Thanks guys. Always better to get a second set of eyes on a conundrum.