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1977 MGB droning sound

Does anyone have a clue what may cause a rhythmic low frequency droning sound in a MGB? Most evident with top up it starts about 2000 rpm, very evident at 3000 and up. Going down hill, throttle off, still at 3000, sound diminishes. Cruise or throttle on the rhythmic droning is evident. Some time ago it was suggested to tighten the exhaust support bracket connected to the bell housing and that actually helped. The MGB as a unibody car could turn the whole thing into a speaker so any vibration may be audible. Parked and hood open it still produces the sound. Just curious

Car Model MGA/MGB

Re: 1977 MGB droning sound

It was suggested,as a simple test to remove the fan belt eliminating both the water pump and alternator from the system. With the sound gone, I replaced the working alternator with a spare and the noise issue is eliminated.

Car Model MGA/MGB