Audio system causing knocking sound?

Recently when my car is started there is a very distinct “knock” coming from up and behind the driver’s seat. It sounds exactly like someone is knocking on the window (once or twice). It happens a few seconds after the car is started up. The first few times I heard it, I literally though someone was knocking on my window to get my attention.

Then, when the power is turned off, there is a deeper, more of a thump, sounds from the center of the roof. This one sounds like a deformed metal sheet that is popping back into place.

Within a week of first hearing these sounds, every once in a while the radio would turn on or off strangely. For instance, when turning the radio on, it would come on immediately, then silence for a second or two and then it would be okay.

So these two problem may be unrelated but they started within a week of each other, so its pretty suspicious that they are related (or have the same underlying cause).

Any thought on how I could diagnose this or fix the problem… other than taking it to a witch doctor?

Well, you didn’t see fit to share any information regarding the make, model, and model year of your car, so this reply (and most others) will have to be a shot in the dark. But, here is my theory, for what it is worth:

IF your car has a power-driven antenna for the radio, all of these symptoms could come from the same thing, namely a problem with the motor that drives the antenna up and down, and/or a problem with the connections to the antenna. And, if this is the case, then it is obvious that the noises are not coming from exactly where you believe that they are coming from.

Cars made in the last few years have finally done away with these potentially problem-ridden devices, in favor of antennas embedded in the rear window glass. If your car is a low-end model, it might even have a regular old rigid mast antenna. But in the absence of any information regarding your vehicle, I don’t know which type of system your car has.

So–IF your car does have a power-driven antenna, I would suggest that you have someone observe what happens when you turn the radio on and off. Does the antenna extend smoothly and completely? Does it retract smoothly and completely? Does it extend and retract silently, as it is supposed to do? If the answer to any of these questions is “no”, then you need to look at the electrical connections for the antenna and the motor that drives it.

If your car does not have a power-driven antenna, then my theory is useless–but at least I gave it my best shot in the absence of vital information.

Sorry about the missing info.

I appreciate your quick reply… but the car is a 2003 Saab 9-3, so no power-driven antenna.

Okay, here’s another shot in the dark. IF your car has a sunroof, the noise could be the motor and/or solenoid for the sunroof cycling on and off. Does it have a sunroof?

While this scenario would not explain the radio problem specifically, if you have some problems in the electrical system (body control module perhaps) that could account for various electrical components acting strangely.

So, let’s try some basic troubleshooting by elimination. Do you get the noises if you have the audio system turned off when you start and shut off the car? If so, try it again after pulling the fuse for the radio. If you still get the strange knocking and metal popping sounds with the radio fuse pulled, then I’d say it is extremely unlikely that the audio system problems are related to the other noises.

If, on the other hand, the noises go away when the radio is turned off then if may just be the radio is the culprit. Failing amplifiers can put strange electrical pulses out to the speakers during power on and power off. In modern radios this generally means it’s time for a replacement. Car radios are just not worth repairing, unless it’s a high end system, and even then it may be cheaper to just replace the head unit. For sure on most factory car radios it’s cheaper to replace than repair.