can't be sure but the culprit for the hissing sound could be one of below (or could be all 3 together),
- Input shaft bearing
- Pilot bearing
- Throwout bearing (or some people call clutch release bearing)
Logic explanation, you mentioned that the noise goes away when you free the gear while moving, i would overrule the propshaft since the sound is there and most noticeable during accelerating and decelerating (means when the clutch is engaged). the bearings that are under most load are the pilot and input shaft bearing. Throwout bearing problem is usually more noticeable when you disengage the clutch.
You're pretty much safe if the the problem is the pilot bearing (can buy at any bmw spare part shop) or throw out bearing (this bearing comes along when you buy a new clutch kit).
However if the input shaft bearing is shot, I believe this cannot be replaced since spare part shop don't sell this (tried looking for it at bavarian auto and they don't have it

). your only option is to change the gearbox. I seldom hear people overhauling bmw manual tranny in Malaysia. So better be safe than sorry, if the culprit is not the pilot or throwout bearing, replace the gearbox. You don't want to spend thousands just to see the problem coming back to you sooner than expected.
Though after all that, we cannot overrule the synchros in your gearbox are shot as well and if those are shot, you have no choice but to buy a gearbox. Symptoms of shot synchros are usually hard to engage into gear and sometimes whining/grinding sound during acceleration and deceleration as well. So if by replacing the pilot and throw out bearing doesn't solve the issue (ensure that you replace the transmission fluid as well), better start sourcing for a new gearbox.
Pilot bearing is cheap. 30-50 bucks i think and you can get it brand new at Bavarian Auto. But the cost to bring down the gearbox depends on the workshop that does it for you. can range from 200-500 bucks. But since you're taking down the gearbox, you might want to consider changing the clutch kit (if you buy new, will come with the throwout bearing as well), pilot bearing, check the flywheel condition, and just to be on the safe side, replace the flywheel oil seal all together to save you future trouble. all in all will probably set you back at about RM1500 (estimated).
As an extra, check your transmission mount as well. Problems can appear if the mounting is shot as well. But usually it involves hard to shift.
Good luck.