Did further checking. Effect is not present when using elderly USB sound card. Tried equally old Soundblaster from spare computer and results had 'thumps' present . Changing card to different PCIe slot didn't help.

Listening more closely, realised that there are definite drum-fills among the thumps! Faint, but there on vla001.flac. Did a stem-split using BIAB and they can be heard, again faintly, on 'Residual' track at around 28" and 45" and possibly elsewhere. The program 'Acoustica' - in 'Remix' mode - splits the tracks with this drum sound more prominent.

My latest theory is that there is a small amount of crosstalk from a drum track on the viola one on original recording. Also, the USB sound card, which didn't produce these noises, is 'gating' very low-level sounds that Soundblaster, with perhaps greater 'noise-floor', is still playing.

Anyway, I'll send the various tracks to PG to see what they make of them!