another noise

Ok here goes , Yet another noise! how do you go about explaining a noise in words .
Range rover p38 1999 2.5 dse Auto.
when im driving just before the auto box changes gear the engine noise gets louder than normal . Its a bit like somthing vibrating !!
It happens at between 2000 to 3000 rpm in all gears, as i just said as the engine is at its most revs before the next gear kicks in.
If a window / sunroof is open you cant here it so much, but thats probabley because of the extra noise from out side .
The car drives well , it changes gear nice to. I have put in nautral and in all the gears manualy and it still does it. cold or hot no diferance either.
I have checked all parts that could rattle like heat shields and gearbox cover Etc.
my thought was gearbox , but if it does it in nautral this cant be or can it?.
Just done an engine oil change as well and no change.
Also this may not have anything to do with the noise but the aircon belt was missing when i bought it and i havent replaced it yet.

Loose baffle in the centre silencer?

Thats one had not thought of , I will have a look.
Cheers

Been following this thread, but there doesn't seem to be much happening. My Rangey has a similar noise, when it's cold and even when its warmed up, the engine seems noisey between 2250 and 2750 rpm it only does it when driving (under load) and it seems to disappear when the engine is well and truly warmed up, say after 30 minutes or more driving. When revving the engine with the bonnet up and door open listening, the noise is not there. I don't think it's a transmission noise as it is present at the same rpm in every gear, but i could be wrong. Any ideas?