Strange rear of the car low frequency rumble when letting up on the gas

[Pat Martone]

I’m at a loss in discovering the source of a low frequency rumbling sound coming from the rear of my ‘31 Slant sedan, when I let up on the gas pedal. This is not an engine or exhaust noise. I have checked/adjusted the tailpipe muffler clamp several times, so I don’t think that is the source. I’ve even wrapped a piece of high temp silicone rubber under the tailpipe clamp to isolate any tailpipe vibration from being transmitted to the frame. I welcome any thoughts on the cause of this annoying and prominent sound. Thank you.