There was some revision v1.04 servos released that would not work in multiples driven from a single channel source with Futaba PCM. Sounds like you have two of these servos.

I'd suggest you send the servos to Hitec Service for a component update to eliminate the OEM compatibility issue that was later realized and resolved. Or you could drive them elevator halves individually with separate channels and a mix, BUT you’re limited to seven channels with your RX. Or you can use a wye which is boosted and or amplified, this wye will isolate the servos from one another, this is the least desirable IMO.

There is no charge to update your servos. Simply include information with regard to your problem, ask them to update the software and perform the component upgrade for Futaba applications. Don’t forget to mention one servo needs to be reversed.

The 5995TG maybe reacting to noise or simply working as designed, premium digitals tend to buzzz-chater-beep when loaded. Try and remove the servo arm note if the servo still chatters, make sure there is no binding and the static load is minimal at neutral. You might even remove all the other servos from the RX and note if the noise your hearing is still present and being caused by some sort of electrical feedback.