Well from a visual inspection my speed daddy headers on my 04 GTP seem to fit fine. I even beat in the firewall a bit so the O2 sensor could pass. Now this isn't a big issue but it still bothers me a little. Whenever I go in reverse, put a bit of power on it I can hear the car slow down a bit and make a noise (grinding like sound). The car runs fine, and there is no real evidence of my O2 hitting, only in areas where I beat it. I checked clamps, etc, and everything has surplus room! I never did bend the transmission dipstick, and im wondering if when the engine goes in reverse if that rear header isn't smacking into tranny dipstick? There is a bit of clearance but its the only thing I can see as the possible issue @ this point in time. Spent a good hour out in garage beatin the firewall on 90psi and got it really dented up. I can install the O2 so much easier now and I can see a good gap between the two. Just not sure what is causing this, any thoughts, am I correct with transmission dipstick maybe?

When driving the car, I get KR @ the shifts, but that is it. Otherwise it runs KR free.

The dipstick is mounted to the transmission and the transmission is mounted to the engine, and the headers are mounted to the engine. so they really can't flex and hit. Any place you think it might hit, put some play dough. That will tell you if it'* actually hitting.

A header clearance issue won't make the car slow down and I've never heard a grinding noise with a clearance issue either. How'* the tranny? You might have an issue there with reverse and not a header issue.

The transmission was replaced in my car less then 3 weeks ago. I had an issue with my stock transmission and they found metal in pan. I got the new used transmission installed by dealer for free because I was under warranty. I'm a bit concerned about taking it back any time soon because they will recognize I now have headers. I'm thinking about waiting some time before doing it. I have a year on the warranty from the dealer and warranty company. Also first time they didn't send out an inspector, but GM dealer told me that if they did, they would have ended my warranty right there because of the modifications. I came in with Cat codes deleted, intake, and catless downpipe. Told me I was very lucky they covered it. It wouldn't surprise me if this shady warranty company (Autoservices out of Alabama) didn't screw me over.

A grinding sound sounds more like a drive train issue. Does the sound change with speed? Check over the axles, make sure nothing looks out of place. Check motor mounts. If one is bad, like the back one, when in rev the engine can shift out of place.

That is sad if motor mounts are bad @ 84k. You would think they would at least construct them to last a long time.

they dont do that anymore they make it so stuff breaks and you gotta pay to fix it lol...thats why the newest car ive had was a 2000

LOL. Man with my modded/ported L27 in the Regal (N/A Series 1 3800), I SHREDDED engine mounts. Within 12 months, I went through 8 torque mounts, 2 rear passenger mounts, 4 front passenger mounts, and one transmission mount. Granted, I abused it, but still, check your mounts. With the L67 now in the Regal, I already went through one passenger torque mount and one lower passenger mount.

hehe i destroyed my front shock mount and drivers trans in the ssei haha in about 3 months or so

I did my front trans mount in. It'* a solid case 8 1/2" bolt mount now. The dog bone mounts work great, but the bushings are a bit weak. A lot of people will flip them 90* to help hold the engine in place better. The rear trans mount is a joke. Check them over. Open the hood and with the engine off, trans in park, push the car back and forth. You will get the engine rocking. A little movement is normal. If you see a lot, start looking around for the shot mount.