My remote started making a high pitch noise. After reading some other posts, apparently the noise comes from the Inverter used to drive the backlight. This occurs when the battery is at full charge (Slightly overcharged). Use the remote for a bit and it will stop doing this. What I have been doing is using one charger base and switching out remotes when the battery dies. I never really understood the problem before, just noticed it came and went. I wonder if someone has come up with a fix to prevent this altogether.

I've never noticed this, but it has been a few years since I had my hands on a TSU7500.

It's possible that the switching frequency has dropped due to component aging. At high battery charge the inverter duty cycle will be a little different, possibly resulting in more audible output. Finally, I've seen some switching power supplies where the inductors are coated with a paste to reduce noise. This paste can decompose over time.