2004 BMW 330xi Issues

Hey everyone, this is my first thread. First off thanks to anyone for any input they may have, much appreciated. Here is my problem. I have a 2004 BMW 330xi with harmon kardon set-up. no navigation, all factory sound system. factory amplified. I am having a kenwood kvt-717 mobile video professionally installed. I recieved it back and everything seemed fine, until i turned down my stereo. With my stereo all the way down but powered on i hear a whistle or whistling noise kinda like a supercharger and it is in corelation with when i depress the accelerator. whenever i'm at a complete stop, it doesn't make the noise. i took it back in and they are looking over it. i have talked to some people in the install business and they are saying it is either a poor ground connection, or i don't have the little piece that comes with the wiring harness for the deck that drowns out engine noise(well forget that I do have that piece when they checked) so now my car is sitting in there shop and they are trying to figure it out. I have purchased everything required, however there is a piece called the pac2 device on (crutchfield)

https://www.crutchfield.com/S-iANnD8COp5l/cgi-bin/prodview.asp?I=127OEM2

which is used to convert aftermarket recievers to factory amplified sound systems such as quite a few luxury vehicles have. they didn't install that piece because they said i didn't need it. you think this could be the issue? peripheral electronics have a piece similar to this that i purchased it is a 4 channel or i should say has 4 adjustments on the corners looks like for speakers( not to sure). I know for a fact that it is the deck and not the car (thank goodness) because when i turn the deck completely off i don't hear anything. i pulled out the deck and listened thinking it was the deck and didn't sound like it was from the deck, but from the speakers, i dunno kinda odd to me. sorry this is so long, wanted to make sure i covered everything. anyways any feedback would be much appreciated. Thanks everyone.

it sounds to me like alternator whine most likely cause by a poor ground connection.