RCA 1YB-11 rumbling sound

[Dubis7]

Making major strides on my tape cartridge recorder tonight. I have it playing again! Audio comes through fine, and the mechanism seems to do everything it should at the right speed.

However, I'm getting a rumbling nose through the speaker. It comes and goes, but seems to reliably come through as it warms up, then gradually comes through less and less as it's on for awhile. This doesn't affect the audio quality, which seems to drown it out when playing.

Youtube link because I don't think I can upload videos here: https://youtu.be/WcWWbE7dVQI

Also, in the future, should I just make one overall thread for a single project? I know I've been periodically making new threads for individual questions, but it seems like maybe that's not the best option. Sorry if I've been flooding the forum.