I dunno if this is gonna help you guys but i have a '91 fastback and i two have expericence the annoyance of the sunroof whistling... you can solve this by maybe opening the sunroof? yes noise will still come in but it won't be whistling.... But this is wut i did to stop it.... couple of weeks ago i was reinstalling the sunroof on because i have taken it off on a hot day, so when i tired to install it back i did it i correctly and the damn latch broke... yes i tried the tacky way of fixing it by super glue, and no it didn't work cause the latch is under pressure when closed... so i called up nissan and they quoted meit would cost 50 buck, i was like wtf for this piece of plastic???? anyways there is this used nissan part shop near my house and they had an '89 sunroof which is really simalar to the 91 (maybe even the same i'm not sure) but i bought the latch for 20 bucks

(better than 50 right??) when i installed it it was really surprised on how loud it was when i closed the sunroof... so i took it for a spin around the block and did 110mph to see if it would whistle, and long and behold it didn't whistle a bit... i was so freaking happy cause i'm usually going 80mph and that's when the whistling starts... so maybe this might help u guys???
Oh yeah the hardware that i got from the '89 (rubber spacers were a bit thinner than mine so maybe that's how i solved my problem.... and be careful about ur latch to not put so much pressure on it cause it might break....