M6 Transmission rattling at speed?

[the Swede]

I've been going through all threads here, but haven't found this problem mentioned, so either I'm alone with it or else I'm alone being too sensitive?

Travelling 55-75 mph (I've never been going faster since I got my $1000 fine...Thanks, Virginia troopers) there is a high pitch rattling noise from the gearbox at medium load. Very irritating, and it doesn't matter which gear 4th, 5th or 6th, it's just the same so doesn't change by rpm. (Maybe 3rd too, but then my Flowmasters are drowning any other noise anyway) Coasting makes the sound completely disappear instantly, more load (acceleration or going uphill) also takes away the rattle. Doesn't change if I hold the shifter, so not related to the linkage.

Hate taking it to the local dealer - they wouldn't notice if the entire transmission was missing anyway. Should I try switching transmission fluid? Anyone with an idea?

[camaro2k10]

When you find out buddy let me know because the dealership replaced my whole gas tank saying it was a piece of it broken off inside of the tank and i knew they were complete idiots because it didnt work and wouldnt make this noise