I have this horrible squeaky noise coming from my rear deraileur area. It only happens when I pedal. Watch below for the video.
Sounds like jockey wheel squeak to me.
Although keeping your bike clean is generally a good thing, it does have some negative side effects like washing away grease and lubrication on your moving parts.
As part of your regular maintenance tasks you should be periodically replacing the lost/contaminated lubrication from your moving parts, in this case, likely to be your jockey wheels. Here is a GCN Video on how to service your jockey wheels
Clean and lube the chain. If that does not work service the jockey wheels.
Here's Park Tools page for derailleur service: https://www.parktool.com/blog/repair-help/rear-derailleur-overhaul
If/when you do take the jockey wheels out, keep track of which is the top and bottom wheel and which way around they go. The top and bottom wheels are different and some derailleur's wheels have a direction of rotation.