Hello. Firstly, welcome to the forum.
It's possible your gearbox bearings are shot. Your Bravo (along with most other T-Jets) is fitted with the infamous Getrag M32 6-speed unit and has caused no-end of headache for sufferers of the problem. A load of Vauxhall models are also fitted with this box so there's complaints amongst their communities too.
This has been covered extensively already, so I recommend you do a search on this forum for simply the term "M32" and double check your symptoms match up with your search results. I had issues recently with my M32 gearbox but it caused whining in gear 6. If I recall correctly, gears 3 and 4 are on a different shaft.
Don't worry, if the bearings are the cause, they can be replaced relatively easily by a transmission specialist and it shouldn't cost more than £400 or so.