If your classic car makes a grumbling noise when warm, first make sure the carburettor mixture is correctly set. Then, try retarding the ignition timing a little at a time. Growling or vibration is sometimes symptomatic of the pistons pushing down on the crankshaft before the crank pins have gone over centre – and this ‘frustration’ is likely to be more evident in higher gears where the crank has to push harder against the inertia of the car, and is known as ‘Pinking’.
If the noise goes away, check the timing marks. If it seems to be too far retarded, a fault in the centrifugal advance mechanism may be causing it to advance the timing too much. Twist the rotor arm against the pressure of the bob-weight springs. It should turn no more than about 15° and then stop. If it can be twisted by much more than this, then it’s probably wrongly assembled.
If the engine checks out as OK, it’s possible that there’s relative movement of the main- and lay-shafts of the gearbox. This may be due to a worn bearing – or perhaps an end-nut of one of the shafts has worked loose.