I have a loud muffler-like sound in the cabin as soon as I go over 50mph. I have dealt with this noise for well over 60k miles but seems to be getting worse. I can’t pin point where the noise is coming from; front, back, under-carriage. I installed sound dampening pads in trunk and under rear seats, but had no effect in reducing the noise.
While doing some preventive maintenance, I used a can of CRC GDI intake valve cleaner according to the instructions, and while in the process, the car was releasing a strong sulfur (rotten eggs) smell, but no white smoke at all.
1. Has anyone experienced this sulfuric smell while using CRC GDI? Is this typical of using this product?
2. I read that this sulfur smell may be an indication of the catalytic converter going out. Has anyone had a catalytic converter that needed replaced? If so did you experience some in-cabin noise?
The car runs well, accelerates without a problem, and I have no lights-on on the dashboard.

 

I have no experience with the crc valve cleaner. I'll assume it will create some strange fumes, since you are adding something combustible and it could be the smell. I wouldn't replace the catalytic converter unless you have a P0420 code or if the catalyst material has crumbled. You'd have to pull the converter to look inside.

Also, if the car doesn't have the rotten eggs smell during normal operations, I wouldn't worry about the converter.

From your description, you might have an exhaust leak. One method I've seen is where on a cold exhaust, spray soapy water along the way from the exhaust manifold to the muffler. You hook up a shop vac backwards to push air into the exhaust. Then look for bubbles from the soap to locate any exhaust leaks. There are some youtube videos on this.

 

So I finally found the culprit of this rumbling noise. It was one of my rear wheel bearings. I was able to change the wheel bearing hub and the noise is gone.✌️