A bit like the infamous Kingsway Hall 'rumble' due to the Picadilly Line trains running underneath. All beautifully captured by Decca.
That is often a problem of 'live' recordings made in concert halls, churches etc. There are unwanted contributions from air conditioning; heating systems; traffic outside the venue, and sometimes from the poor acoustics of the venue itself, especially if care has not been taken with the choice and positioning of the microphones.
BTW Tom, which Vegh Quartet Beethoven recording do you have? I have a boxed set; but I'm not at home at the moment to check the recording label