Loud Exhaust on Hills

[Mr_V]

I have as 2005 Subura Baja Turbo with 80K miles. Recently, the check engine light came on and I started having trouble putting gas into the tank. It acted like the filler tube was blocked. A local mechanic checked the code, P-1433, and replaced a “vent valve” and the problem went away. Now, when I drive up an extended hill, after about two mile the car shakes a little, like a CV joint going bad, than starts to sound like the car has headers. The exhaust becomes loud, like a hole in the pipe, but the car doesn’t loose power. After cresting the hill the sound goes back to normal after a few miles. Once when this happened, it sounded like air was being blown into the gas tank and I could hear the gas bubbling. The check engine light has not come back on. I talked with the mechanic but he has no idea what could be causing it. I’ve also lost about 2 mile per gallon. Got any clues? Thank you! Mark V