We  are digitising 60 year old reel-reel tapes.

For this we are using an Akai DS40000 Mk 2 deck, a Behringer USB
digitiser, and Audacity. This is all connected to a Toshiba Qosmio G32
running Ubuntu (Linex).

The tape plays smoothly with no sticky-shed or squealing.

The recording was originally aired by the BBC on FM in the 1970s.

The timeline is BBC Transmission > Radio FM Receiver > Reel-Reel
recording > Playback on same tape deck > Behringer digitiser >
Audacity > Saved as FLAC file.

However the resulting digital recording has strange intermittent
'warbles.' It is as though the tape was momentarily caught on

This is an example. Yes - some do like this kind of music!!! But it is
the quality of the digital recording that is of concern.

Any ideas please as to cause?

Thank you -

Chris B.