Diagnosing the same noise related issue with two bikes .. any ideas??

Have an ongoing weird issue with two bikes and I thought I'd throw it out to the hivemind here to see if anyone had any ideas..!

Bikes are both Merida Crossways (male and female versions) - both have developed the exact same issue.

Have owned both bikes for 5 years, issue has started on both bikes around the same time in the last two years.

Sometimes, without rhyme or reason, we'll start getting this noise from somewhere in the rear wheel section of both bikes. It happens randomly, we'll be cycling and one bike might start doing it, then the other bike. It'll also randomly stop too.

It only happens when peddling. You'll be peddling along and you'll notice a high pitched noise starting behind you. The noise gradually gets louder and louder. It's not really a grinding, more of a high pitched squeal. It only happens when peddling. The moment you stop peddling the noise stops… until you start peddling again and it resumes.

Sometimes the noise will stop if the bike takes a bit of an impact, like a curb or a pothole.

Usually when this issue happens you can't freely pedal backwards, the chain catches in the front derailleur. I thought there might be an issue with the derailleur misaligning, but then that wouldn't explain the random occurence of the issue.

What's really annoying is, when the bikes are stationary, the noise doesn't happen. So I bring it into a bike technician to fix it but can't show them the issue. They've replaced the chains, the whole gearsets - issue eventually comes back.

I was thinking a rear disc brake rubbing maybe but then it wouldn't be only happening when peddling.

Last technician thought it was maybe squeaky pedals but it's definitely not the correct 'sound', as it grows in intensity rather than just being a squeak (plus the issue with the chain catching when peddling backwards wouldn't be the pedals).

Any ideas?? Feels like we're going mad at this point haha.