The car is a 1971 142E with M41 with an IPD header and Simons exhaust system.

I have done a complete restoration on the car and as I have gradually eliminated most of the 'unnatural' noises following reassembly, I am now left with one persistent sound that I cannot diagnose. It seems to be originating from underneath the floor pan in the OD area or perhaps farther back. The noise occurs when the car is in gear and accelerating between approximately 1600 - 2400 RPM.

The noise sounds exactly like what I would expect from the knocking of the exhaust system on the body. However, when I grab the cold exhaust system parts and shake them there is no banging. Visual inspection doesn't show any contact points and in general there is lots of clearance between the exhaust components and the body.

I initially thought perhaps some kind of clutch engagement issue, so I did a test drive, sped the car up in second gear, let it coast down in second until the engine speed was about 1600 RPM and then reaccelerated. The noise was present so it is not related to the engagement of the clutch.

The noise is present in 1st, 2nd and 3rd gear at the same RPM so I don't think it is a drivetrain noise.

The rubber engine and transmission mounts are intact.

I am at a bit of a loss - any suggestions?