i dont run the LSXrt but i do run the LSXr i tried fitting the 90mm tb but on the bottom it barely covered the O ring i mean barely, i had a whistle probem it was the slightess whistle but it was a whistle,, couldnt find it for nothing, once i got the 102MM the whistle stopped, not saying the TB is your problem just what i went through

 

I have heard some whistle like skeet mentioned,mine doesn't do it,it growls more than anything.LOL
When i had a plate,it did whistle,but that was not on this manifold,so that doesn't count.
I never had a problem with the TB not sealing on the o ring,worked perfect on mine but that doesn't mean it can't happen

 

Thanks for the help guys i think it is ok going to just run it and hope for the best.. lol....

 

Mine has a slight whistle around 55mph so around part throttle. I think it could just be how some of them are I wouldnt be to worried about it.

 

chris no worries, my fast whistles also and i have yet to find anything wrong. Tried pretty much everything you listed and got the same results. maybe its just the intake doing its thing? my brother also had a fast on his trans am (car version) and it also whistled like crazy. He gave thunder racing a call back and they told him all vehicles they install the fast on have the same noise. good luck on your runs and let us know how you do.

-Jacob-