I got the truck back on the road last weekend, after my winter/spring overhaul attempt. It drives well, but I'm getting this strange hum when I'm driving that persists until after I turn the key off, and then it fades away.

Exhibit A (from this morning):


At first I thought maybe it was the stock horn I just hooked up. But I pulled the power from that, and I'm still hearing this.

What is my truck trying to tell me?

 

I tried to watch and listen, but it's tough to hear anything. Do you still have a vacuum wiper? Maybe a leak there?

 

Yes, I still have the vacuum wiper. I'll check that.

It's funny about the sound on the video. I heard it fine when I played it back on my phone before I uploaded it, but I can barely hear it through my laptop.

One more thing. If I turn the engine on, let it idle and then turn it off I hear nothing. I have to drive it before it starts humming.

 

heater motor?
As Shane said, hard to pick up.

 

The heater motor is disconnected. So it's not that.

 

What is the configuration of the truck? Mods? You're not boiling the battery, are you? Doubt if a generator would cause the battery to boil quickly.
How about air venting into the gas tank?
Edit: Radiator cap venting?

 

Hmm.. What if it's an older battery that's been discharged many times?

Flathead V8, running 12V. Just completed a front disc brake conversion. 9" rear end. Other than that, it's pretty close to stock.

 

I dunno, but I think it's in "G".

 

On your brake conversion, did you chance it to power brakes? Power boosters will sometimes make that kinda noise. Good luck.

 

I was just working on my truck and realized the radiator cap venting could cause this noise.

 

I can't imagine anything but the vac wipers being continuous.

 

That's a strange one. The tone has a hard edge, not like a cracked/loose vacuum hose would sound. It definitely is a built up pressure that is leeking down. Electrical systems will bleed off but I can't think of any electrical system that would be in your truck that would have an audible component that would make that sound. Cooling systems operate under pressure but if there were a leak in the cooling system there would be water associated with the leak. That is unless there is an air pocket in the system. Since the sound seems to be coming from within the cab I would look at the heater core or at least cycle the heater to ensure that there is water flowing freely through it. This is a strange one, let us know when you find it and what it was.

 

The heater core is bypassed. It has a leak, and since I didn't drive it this past winter, I never got around to having it fixed.

I just got back from a short trip. When I stopped, I got out and then I shut it off. If I heard correctly, it sounded like it was coming from the general direction of the radiator. When it cools down some more, I'll check the coolant level.

I've been running an aluminum radiator for over a year now, and one thing I noticed is that it seems to hold a lot more pressure than the stock rad did. My heater core sprung two leaks since I installed it, and I've been wondering if the increased pressure is the cause.

 

!! What # cap are you running?

 

If you don't have an overflow tank, the noise you hear is the air returning to the radiator where the coolant had been forced out when hot. What you hear is not abnormal.