I was measuring my exhaust tips on my Flowmaster American Thunder axle backs and when I looked down the tips I noticed a small - slightly smaller than a quarter - rust hole.

There is no rust or anything on the outside of the muffler other than usual road wear, and they still sound great, but I don't know about inside the muffler itself.

Should I take them off and use something to clean out the inside? Should I not worry about it?

Thanks

 

Flowmasters are pretty notorious for rust from what I've heard

 

The hole is where?
In the tip or the pipe attaching the tip to the muffler?
I would not 'clean them out' as introducing more
water to do so would cause more rust.
Do you not get her up to temp everytime you take her out?
Short trips deposits water vapor in the exhaust and not getting
the exhaust system hot enough to get rid of it causes rust.
I guess Flowmasters are not constructed of Stainless Steel?
I know the flowmasters are not stainless steel....

 

Flowmasters tend to rust/corrode/what-have-you. If it were me I would probably get new mufflers, seeing as how winter is coming and salted roads will follow.

 

Its in one of the muffler chamber walls - you can reach down the muffler and put your finger in it

What I meant by "clean them out" was to run through some sort of rust protecting fluid or something - not water haha

Now that I know Flowmasters are not stainless on the inside, I am very disappointed and don't think I will buy anything from them when I look to upgrade.

 

Maybe a drain hole?

 

No, it's obviously a rust hole

It isn't in a drain hole location or even a place where water could accumulate

 

Maybe spray a little of some rust converter product onto it until you can pick up some 40 series or something.

 

I love my American Thunders' sound. They're Series 60 I believe.

Got a recommended product?

I think I'll do the entire inside of the muffler by running a small flow through the mufflers while tumbling them slowly

 

Aluminized steel Flows don't rust any more or less than any other steel product. Looking at their website, the only non-stainless late-model Mustang part they seem to have is their 17410, for the 05-09 GT, and the 17421 for the same year V6. Those are both available in SS, and all of the 11-up parts appear to be stainless, as well.
If you're concerned, you could grab some POR 20, which is a rust-proofing high-heat paint- just Google it- and paint the mufflers inside (as much as possible) and out.
Like someone mentioned, too, drive your car more! Another option is to drill weep holes into the mufflers. A single 1/8" hole right near the back wall will let moisture drip out of the muffler, and helps a lot in stopping rust-through from the inside.