Ok..this is about the weirdest thing I have seen in quite a while..and believe me..I get all the weird stuff.
The story is that a customer came into my shop after another shop added an amp and sub to the vehicle. He stated it had noise so I figured a simple ground loop problem...charge the guy 50 bucks and everyones happy. But not this time.....
This humm sound ONLY occurs when the vehicle is at idle.(900 rpm or so)..and at around 1000-1500 RPM. It completly goes away if the idle is 50RPM below idle and anything above 1500 or so. Keep in mind this is NOT alternator whine or any kind of engine noise...it is a hum...around 60 -70hz. Now the noise also completly goes away when you open any door. Close the door and it creeps back in.
Now I tried all the usual stuff...changing out the LOC.(tried several types and brands)...regrounding...running ground to the battery..running power outside the vehicle..tapping the LOC before and after the factory amp., soldering RCA;s off the preamp side..experimenting with resistors..inductors...shielding...ground lifts...ground adds..different amps (went through 3 different amps with no change)...different sub box (didn;t think that would help but wanted to eliminate all possibilities) no change. I even tried a sub-sonic filter and still no improvement although the tone/level would change slightly... which leads me into believing this "hum" is in the audio itself somehow through the head unit which has all kinds of lord knows what integrated into it with this cluster%^*k of a vehicle.
The vehicle (a 2007 Acura TL type R six speed) has nav...reverse cam...voice recognition..blue tooth..basically loaded to the hilt with factory over enginnering. But I see no reason why an amp and sub should be such a problem..it's a glorified Honda for jeepers sake.
But heres the real clincher.....although I don't know what is actually causing this...I found a way to make it stop...at least until the vehicle is shut off..then re started. What happens is exactly this...I get in, close the door, start the car and wait until I hear the hum. Then I open the door, the hum stops..close the door...the hum starts again. I open the trunk and can see the subs moving and hear the hum. I hoover my arm around the sub enclosure (the amp is mounted on it as well) from the side and top of the enclosure back and forth and like magic...the hum completly goes away. Literally like magic..but it is not magic. It happens.
I have never seen anything like this before but there must be a logical explaination. For now I simply put a switch on the amp remote wire up front so the customer can turn off the hum (it goes away while driving and only returns at those specific RPM's) but this is just a band aid for now.
I requested he leave me the vehicle for some time to perhaps find a way to shield this noise out because somehow waving my arm in the rear area of the vehicle does cause it to go away..but I would hate to spend all day on this and not be able to solve it so....if anyone has ever experienced this issue before..or has any ideas what this is I would really appreciate it. The only other thing I ask is please don't ask me to check grounds and basic stuff like that..it's been done to death on this car.
Thanks