I have a 2001 Infinity QX4. While driving slowly a rattling noise started in the engine area. The noise continued when the car is parked and idling. (audio clip 1). I opened the hood to look but did not touch anything. I did not see any obvious problems. I then closed the hood somewhat forcefully. The noise stopped!
I drove the car about 10 minutes with no noise. They next day the noise recurred, again when idling. This time the noise did not occur in “Park” and did not occur in “Drive” but started when I put the car in “Reverse”. The noise happens when car is in “Reverse” and not moving or “Reverse” and actually driving in reverse. (audio clip 2). The car seems to drive fine.
The car was recently serviced, including tightening and repacking the front wheel bearings, reconnecting a failed exhaust system connection with a splice (the joint between the catalytic converter and the muffler), and replacing the crankshaft position sensor, 3 ignition coils and 3 spark plugs. Several mechanics inspected the car a few weeks ago before the noise started, and did not find any other problems. The exhaust heat shield was loose and removed a few weeks ago during the exhaust system repair. I think there is only one heat shield but I’m not sure. There are no obvious visible problems with the exhaust system - based on just looking under the car when it is parked.
if you can, put the car on jack stands and go under and shake the exhaust system to see if it is hitting anywhere. up and down and side to side. do it when the exhaust is cold. safety first.
It’s likely the engine/transmission is shifting position slightly, which in turn is moving something else. While a loose heat shield or exhaust component is likely, you may–and bear in mind this is strictly a guess on my part–also need one or more engine/transmission mounts.
