Transmission makes intermittent high pitched noise

97 jetta 2.0, manual, 210K miles
Car will make high pitched noises inconsistently while engaged.
(one time it wiIl pitch high frequency sound; next time I drive may or may not hear same sound)

I replaced the transmission at 150K

When on the lift rack they found that the transmission reservoir was full but looks to have small leak at axis in housing where coupled with (differential?)

I have been to transmission shops in this area and the best recommendation is to wait till the sound becomes more prominent.
This is counter to "better to maintenance than replace" ($$$$), as I have been taught.

Your opinion is greatly appreciated. Thank you, James