Analogue Pocket whistling while charging and on

Yesterday, my AP stopped turning back on, so I took it apart and reattached the battery connector, and it turned back on again and started working. But after a while of it working okay I decided to test if it still was charging okay as well, plugged it in, and noticed that while both on and charging it seems to make a high-pitched whistling noise with 2/3 of the chargers I have available (the fast charge ones) and that I'd been using previously with no issues. There'd been no issues with battery life either.

My concern is that this could be related to the battery. I live in the UK so getting support from Analogue in a timely manner is probably out of the question and I also no longer have a guarantee, so it would likely be incredibly pricey. As I understand it, Analogue also do not sell or send out replacement batteries. So my questions are: - Has anybody experienced this before and was it a faulty battery or was it something else (eg. capacitor)? - Could it be related to (potentially) slightly damaging the wire while reattaching the connector? - Could anyone more knowledgeable than me with electronics confirm that it is most likely an issue with the battery? (And also that I need to get rid of the battery as a consequence?)

I haven't taken apart the battery pack yet to check the actual lithium ion battery inside so maybe I should do that as a next step before I commit to actually fixing it or getting rid of the battery.

(Also, I have seen the posts regarding adequate battery replacements so I have a rough idea of what to do to fix it if it is the battery, but I'm inexperienced and it would be helpful to know if anyone else has successfully done this.)