Spray something over the rubber on the one that's noisy, even WD40 will silence it for a while.
I'd forgotten that yours has SUs which, ISTR, don't have the steel olives between the carbs and the manifold, but the symptom described is exactly the same as I have occasionally with mine, so it must be a leak through a crack in the rubber. I guess that you can't do away with the rubbers on an SU-equipped car since they provide the required distance and adapt the stud pattern of the manifold to that on the carbs, but I'm told that they can be replaced with solid spacers. Those would shut it up and no mistake.
When mine first made this noise, I thought that I had a failing bearing in the alternator, then it became noisier and sounded like a slipping alternator drive belt. Like I say though, something that contains glycerine will soften the rubber and reform it sufficiently to stop the symptom until you get new or replacement rubbers or alternatives. Silicon grease, as would be used for seating the water pump bypass hose in the stubs, would also keep it quiet for a while.