Could it be spark knock?
 

I have a 98 Grand Marquis LS with 126,000 miles on it. It does not use, burn or leak oil, it does not smoke either. I replaced the intake manifold about 1000 miles ago. It has a check engine light that says p0403 i think, which means cat converter, egr or oxygen sensor I was told.

Well heres my new issue. I pulled the carpet from my car to steam clean it top and bottom bc I have the dreaded firewall leak. Now that the carpet is up I can hear noises from the engine that I had only heard once before. Heres how it happened the first time.

I was driving and came upon a hill and my car was still in overdrive. I then began to hear alot of tinging, rattling type sounds from the engine. I first thought that maybe the engine is wore then I thought maybe its spark knock.
Today I was driving around and can hear that rattling noise even better now that the carpet is up.
It seems to only do it when the car is in a high gear.
I heare this same noise in my dads 94 f150 with 5.0 motor. It does it when we pull a trailer.
Any idea? Iam running 87 octane and thought maybe when this tank gets low feeling up on premium and maybe some additive to see what happens.

I have also heard maybe its a cat, but it doesnt sound like broken metal flying around in a cat.
You cannot hear the noise with the hood open reving the engine.
Thanks in advance.

.That's an EGR code. How about with the hood closed. Have someone sit inside witht the doors/windows closed & do the revving while you stand beside,in front, & behind listening for the noise. If the other persone can hera it but you can't then you know it's on the inside of the car. If you hear it then trace it down. If you don't hear it then it may be a prob with the EGR system. The EGR wont operate with the car in park. Below is a link abot the egr system & code P0403.

http://www.obd-codes.com/p0403