Weird highway smell
 

I have a question about an issue I've been experiencing for a while now, and I can't quite seem to figure out the cause of it. I searched all over the forum looking for a similar problem but found absolutely nothing, so I'll do my best to explain this.

If I drive on the highway with the moonroof open, I start smelling something kinda funny once I get up to 75-80 mph. I will note that this is my normal cruising speed on the highways, and there is no noticable smell at 65-70 mph so it only happens once my RPMs hit around 3500 or so (I have tiptronic auto). Also, the A/C is NOT on when this happens, the vent is on 0 setting so it's just simply blowing air through. The smell doesn't come out of the air vents, because I don't smell it when the moonroof's closed ... only when open.

Now to describe the smell is kinda difficult, and my brother said it smells a lot like a worn clutch, I wouldn't really know since all of my cars have been automatics. I think it smells like rotten eggs to be honest, but that's kinda weird in my opinion ... Anyway the brother advised me to change my tranny fluid, and I had it fully flushed at a dealership about 2 months ago with Mobil1 Synthetic. The smell is still there ...

I also recently replaced my front brake pads with PosiQuiet ceramic pads, so I don't think it's my brakes either but I'm really stumped as to where the smell could be coming from. I really do think it's from the tranny, but I don't know why only on the highway? I've been thinking about an auxillary tranny cooler hoping that would fix the problem but I haven't been able to find one anywhere. eBay has some Hayden ones on there but I dunno which one to pick .. there are quite a few.

One last thing I've thought about is maybe coolant? I've heard the car can smell like that because the overflow tank will let the smell out through that hose that is on it. Then while the fan is on, it blows the smell where you can smell it ... or something like that. Should I consider a coolant flush? I have never done it yet and the car is at 36,000 miles.

So anyone ever smell the same problem by any chance?

Welllll.... The moonroof creates a vacuum in the car pulling in air from any low resistance area....

Check the boot around the shifter (pull the cover) and inspect for tears/holes.

Check the firewall for new 'holes' or penetrations.

Lastly, check your hood rubber insulalation - allows air into the intakes if it is not there....