Line-x. There is nock-off stuff for sound proofing, but it will not be as good. It will probably be cheaper to get rid of the loud exhaust. So if you want to keep the exhuast then you will have to either deal with the sound or pay the money and do the work to get the cab to resist the out-side noise level. Dynamat does that and does it very well.

The reason I said to do both is because if you have the interior torn apart anyways, you might as well do it all and not have to worry about it anymore.
Just my opinion!

 

Many things that you can perform in this regard. First of all, you need to check the quality of your stereo system, speakers and subs. If you think they don't have any issue then I would like to recommend the use of sound deadening material. From here ( https://99carstereo.com/dynamat-alte...ning-material/ ) you can the best sound deadening material both Dynamat and its alternatives are available. I have personally used HushMats and found them good especially due to easy to install and affordability features. Though you can use some spray and foams to dead the sound with DIY ideas but to get any sound deadening material from the market.