Brywood

Need help, 1st let me set up what I have installed. 6.5 infinity references in the front and an infinity reference 12 inch woofer, ported box. All being pushed by a PPI 4 channell amp (6004 model I think). sounds great! I had all of the above installed a couple of days ago by a well know/chain audio store. I kept the factory HU. Heres the problem - when the engine is turned on I hear a fast ticking noise coming from the tweeters. The noise is there even with the radio off, but as long as the car engine is running, the noise is there. I took it back to the shop and they had it all day, tested, re-wired, and the decision was the factory HU was making the noise. My concern is that the radio can be on with no noise, but as soon as you start the car, here comes the ticking noise? If you turn up the volume, you dont notice the noise over the music, just when the volume is down. Also, when you turn down the amp's power output to the speakers, the noise goes away, but so does the sound of the music. Suggestions?? I would really like to stay with the factory HU. Any information or suggestions would help, thanks