I have used the search function and have seen others sunroof was rattling too. however there was no posting on what fixed it. I know its the sunroof rattling above 50 mph, as I put a small piece of rubber hose in it and it stopped. But that being done it may not be leak proof as I can hear a bit of wind now with the cover pulled back, thx in advance

 

How cold is it where you are? The reason I ask is, once freezing temps hit. It stays shut until the spring. I've had my Avalanche and h2 come back from the garage and start leaking because the mechanic opened it during winter. By spring they didn't leak anymore.

So if you opened it while test driving it or someone else did this could be your issue.