Aftermarket Midpipe knocking against frame

Fit and finish is going to highly depend on brand of the pipe and some combination of installer skill. You might be able to make adjustments to fix the issue, or the brand of pipe might be just one notorious for fit issues. You might have to get creative with solutions or just have the pipe modified by an exhaust shop.

Unfortunately without seeing the issue, not much we can really suggest other than getting under the car and trying to make adjustments