Difficult conundrum, so please bear with me while I describe it.

Yesterday I purchased a beautiful 2006 Impala LT from CarMax. On the test drive, the salesman and I noticed a slight knocking noise that seemed to be coming from directly behind the dashboard near the back of the engine. The rate is about 240bpm. It stays at this rate whether or not the engine is idling or accelerating.

When we got back to the dealership, the technician took a look at it and it was no longer knocking - me and the salesman thought we were losing our minds! But lo and behold, it began again about a mile away from the dealership when I drove it off the lot.

The technician said it was probably related to the vehicle sitting in the lot for several weeks, but now I'm not so sure. They assured me (an I agree) that it has nothing to do with the integrity of the engine.

My father and I ran several unscientific tests to see if we could isolate the problem. First, we put the ignition in the "ON" position - nothing heard. Then we started the vehicle, and the knocking wasn't heard until 15 seconds later. It seems to not be an issue on start up, but rather it is initiated about 15 second after the start, with no change in engine performance. Also, it stays at the same rate regardless of engine rpm. We tried turning the lights off, with and without the A/C and heat, in different gears, etc, and nothing stopped the knocking.

My father thinks it has something to do with some type of vacuum assembly that pushes fluid or air through the vehicle. I tend to think it may be the sound of hydraulic fluid moving through the brake assembly, which is just behind the dash on the driver's side in the rear of the engine compartment.

I'm at a loss and I'm sure it will become rather annoying, but I love this car. The technician said to drive it for the weekend and if it's still there to bring it in on Monday to have it looked at. Again, it seems to be nothing related to the engine or its components...engine seems fine.

Thoughts? Thanks!

 

Intermediate steering shaft knock????

 

Good call Gwampa, I think you may be on to something.

 

Yeah, drove a rental Impala last year and small bumps sounded like the thing was gonna crater any time and then I got my Impala and started similar noises. Got that fixed pronto...easy fix too.

 

I'm getting it looked at today and will suggest this to them. Hope it's the fix - it's really getting annoying now, like a woodpecker up in the tree.