Transmission - 5th gear resonating

[86 Carerra]

Have an 86 Carerra with 52K miles. During the last 6 months, a constant, loud resonating sound started: it ocurrs when I'm in 5th gear (only), and it starts right around 3K rpm upto around 3,300, then seems to be not so instrusive.

Two good Porsche repair shops have had different takes on it: 1) the alternator charging effect; 2) replace the trans oil with synthetic. This hasn't worked. (I haven't replaced the alternator, yet, as this doesn't seem to be problem).

It may be that the 5 gear assembly, according to the mechanic, needs to be replaced(?!).

Any thoughts on this?