Question: I purchased a 2009 Hummer H3T five-cylinder with 73,300 miles. The truck has an underhood rattle after it warms up, but never when cold. The rattle occurs at just above idle speed, both accelerating and decelerating (about 800-900 rpm), then again at about 1900-2000 rpm. It sounds like a loose exhaust heat shield resonating with engine speed. However, all of the heat shields are tight. The truck has an aftermarket K&N cold air intake system, but I cannot find anything loose with it. There is lots of online help, from loose heat shields to resonant vibration of heater and A/C hoses at the firewall. I've checked all of these, and everything is tight. There is no engine performance issue; just the rattle, like shuffling a deck of cards.

Answer: Three curse terms in a technician's dictionary are rattles, squeaks, and water leaks. Start by isolating the noise. This is best accomplished with a helper. Sounds like you should be able to duplicate the rattle with the H3 not moving. Try holding rpm where the noise is the greatest. It sometimes works better "power braking," meaning your helper blocks the wheels, applies the parking brake, depresses the brake pedal firmly, and raises rpm until the rattle is most pronounced. At this point, work your way around the vehicle and simply follow the noise by ear. Often an inexpensive technician's stethoscope is a great help. In extreme cases, noises will require a chassis ear (electronic microphones placed at strategic points along the chassis, engine, or transmission). Once you've narrowed down the area, apply pressure with a large screwdriver, pry bar, or what have you to the suspect component(s) (no moving parts). If the noise stops, you've found the source.

A few hints:

• At the rpm producing the noise, apply pressure (pry) on the A/C lines leading to the firewall to see if the rattle stops. If the A/C lines look and feel tight, that does not eliminate them as a cause. The same goes for all the heat shields on the exhaust system.

• You say it's a rattle, but if it could also be described as a ticking noise, check for an exhaust leak at the exhaust manifold. The I-5 is known for bad gaskets and broken bolts.

• If the noise is determined to be coming from the engine, shut it down, remove the drive belt, and restart. The noise stopping determines the source to be a component driven by the belt. Noise still there could mean internal engine problems.

• Also, with the engine off, bang on the exhaust system in several areas with a rubber mallet. There could be debris bouncing around inside. Focus on the catalytic converter. The cat's internal element may have broken up.


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