Getting the Skylark Ready for Strongsville

[Machine Gun]

There were a few items that needed attention before my next road trip. Last year I installed new U-joints, replaced the rear axle bearings, and replaced the troublesome carburetor. I replaced a bad front wheel bearing yesterday, and I have to order new door weatherstripping to stop the wind noise that nearly drove me mad on my trip to Buick Gardens last Fall. A new issue came up since then that so far has eluded me.

 

When the car is cold I can hear a low thumping noise coming from what sounds like the front of the car. It disappears when I apply the brakes. After the car is driven for a short while, say around 20 minutes, the sound gradually fades away. I can feel no vibration or other tactile sensation in the chassis, steering wheel, or brake pedal. I'm not sure where to look since everything looks good under the car and inside the brake drums. My first thought was a tire because the noise dissipates after I drive for awhile and the tires warm up a bit, but applying the brakes wouldn't affect tire noise, at least I don't think so. Suggestions?

[NC-car-guy]

2 hours ago, Machine Gun said:

There were a few items that needed attention before my next road trip. Last year I installed new U-joints, replaced the rear axle bearings, and replaced the troublesome carburetor. I replaced a bad front wheel bearing yesterday, and I have to order new door weatherstripping to stop the wind noise that nearly drove me mad on my trip to Buick Gardens last Fall. A new issue came up since then that so far has eluded me.

 

When the car is cold I can hear a low thumping noise coming from what sounds like the front of the car. It disappears when I apply the brakes. After the car is driven for a short while, say around 20 minutes, the sound gradually fades away. I can feel no vibration or other tactile sensation in the chassis, steering wheel, or brake pedal. I'm not sure where to look since everything looks good under the car and inside the brake drums. My first thought was a tire because the noise dissipates after I drive for awhile and the tires warm up a bit, but applying the brakes wouldn't affect tire noise, at least I don't think so. Suggestions?

I had one that might a slight noise and vibration until the tires warmed up, I assumed one was slightly out of round somewhere.  It did change as I applied the brake.  I know you report no vibrations but might be worth taking the tires in for inspection and balancing.

[Ronnie]

I would rotate the tires from front to back and see if the noise goes to the back. If it does that would tell you a tire is the problem.

[EmTee]

4 hours ago, Machine Gun said:

When the car is cold I can hear a low thumping noise coming from what sounds like the front of the car. It disappears when I apply the brakes.

Maybe shoes rubbing an out-of-round drum?  If you have a set of brake diameter calipers you could check them when cold.  There's a runout spec in the shop manual.  If you can separate the drum from the hub you could invert the drum and check with a dial indicator.  Given that the sound goes away when everything warms-up and you don't feel any pulsing in the brake pedal I'd just inspect everything (which it sounds like you have done) and monitor the situation.  Swapping tires front-to-back as suggested above would be a good thing to try, just to confirm or refute a tire issue.

[JohnD1956]

maybe a fan belt that has dried out and now wobbles?

 

[Machine Gun]

Thanx for the suggestions. @NC-car-guy and @Ronnie, I switched the tires front to back yesterday and I'll do a proper road test when get the chance. Weather here is pretty crappy and will stay that way until my wife and I leave for vacation over the weekend so I many not be able to evaluate until our return. 

 

@EmTee, your suggestion to look into the brake drums is one that I hadn't considered. That'll happen in Round 2 if the tire swap doesn't lead me to the problem.

 

@JohnD1956, the belts aren't that old but I looked them over anyway and they seem fine. Of course a visual inspection doesn't always reveal what's happening, but the sound seems to be coming from under the car and not from the engine compartment. 

 

I'll report back after I identify the problem. It'll be a couple of weeks since I'll be away on a grand tour of the South through GA and the Carolinas.