faint precession of sound on vinyl

It is almost always groove-to-groove interference. This is easily verified by timing the amount of pre-echo. If it's 1.8 seconds it is a LP problem. Mag tape print-through is also real, but usually requires storage of the tape for years without rewind, and the timing will vary with tape speed and how much tape remains on the reel.

Pre-echo can be eliminated by wide groove spacing, but that drasticly reduces playing time. So groove spacing is dynamically varied as the record is cut according to the music signal amplitude. As always there is a compromise, and the grooves are kept as close together as possible, accepting a small amount of pre-echo that only audiophiles will complain about.

Rwwear...I didn't know that. Can you explain how this helps.

Deaf_j...The playback stylus does not "read the adjacent wall". It is reading its current groove, but that groove was corrupted by the adjacent groove when the record was cut.

As to which channel is affected...it is horizontal groove modulation (mono) which is the problem. (Vertical groove modulation doesn't affect groove spacing). So, the pre-echo signal will be the mono component of the next groove.

Rwwear...I take your point about rewinding the tape, although I don't see how this would erase any print=through.

However, whichever way the tape is wound the first few seconds of music will be next to the second few seconds, and print-through will be the same. The echo, be it groove distortion or mag tape print through, occurs both before and after, but it only becomes audible before the begining of the music because there is no sound to mask it.