Loud noise when running

It might be an exhaust system leak. Hold a rag over tail pipe outlet while engine idles and see if it builds pressure. Listen for a hissing sound up front. That will be the leak.

I had the similar problem. I simply applied the gasket maker on the exhaust manifold because the noise was being cause by a leak. The gasket maker stopped the leak and the noise disappeared. It could also just be a leak at a different place of the exhaust system

Wifes car suddenly started making a "twapping" sound while running.....sound became most evident when car hit 20 mph and got increasingly louder as speed increased. I checked the car.....she needed new brakes...installed new rotors and brakes on front (as that is where the "sound" appeared to be coming from...sound was definately "rotational". No change in the sound with new brakes. Changed BOTH front wheel bearing/hub assemblies. NO CHANGE. Change driver side front CV axle....inner joint seemed to be "clicking" louder than expected. NO CHANGE in "twapping" sound. Finally gave up and put it on a lift....car on lift and wheels spinning at 20 MPH...NO sound fron the front transfer case, no sound in the rear differential....used a mechanics stethoscope and the driveshaft carrier bearing in the middle of the shaft was "grumbling"...you can really HEAR it quite distinctly with the stethescope. Car has 60,000 miles. Ordered new drive shaft as the carrier bearing (in the middle is not a replacement part). Replaced the shaft....no more twapping sound. It was really hard to pinpoint as there was no vibratiion in the steering wheel, no pulsing in the brakes...but it certainly was there when running and I could almost swear it was coming from right underneath the drivers seat area. So, I had loaded up the parts cannon unnecessarily (hubs and axle)...but it was worth it in the end. I did all the work myself so it was just the parts expense. My shop did not even charge me for the lift time....good shop....I'll be giving them continued business.