I have a 1991 Toyota P/U 4WD, 4 cyl, 22RE Engine (Rebuilt in 2005 after 259,000 miles, now I have put 50,000 miles on the engine).

Recently, when I accelerate to 65-70 mph I hear a low grumbling/low pitched sound coming from the engine. When I let up on the accelerator, it sounds fine. If I push the accelerator down far enough and fast enough I hear that sound again. Otherwise, the engine sounds fine, runs fine. Just had a major tuneup less than 2,000 miles ago (timing, valves adjusted, new plugs, etc...). I checked my exhaust system, I had a blown Exhaust Gasket (front side of the Catalytic Converter) which I replaced with success. The Rear Exhaust Gasket is fine. I checked my Exhaust Manifold by carefully running my hand along it's edges (not touching it)while the engine was running to check for any leakage from the Exhaust Mainifold Gasket. I did not feel any, but I only checked the top under the hood. Any ideas on what this sound could be from?

thanks.

 

It could be the manifold gasket leaking. I would check the torque on all the manifold bolts. The two rear bolts were always bad about coming loose.

Those things are screw in studs - if they loosen in the head, you need to take the entire stud out, back the nut off and reinstall the stud before putting the nut back on it. Then torque the nut to the manifold.

 

Not to be the party pooper....But remove the heat shield from the manifold and look carefully for "Spider Cracks" in the casting.