ihf1000 in 07 silverado

I have the IHF1000 installed in my 07 new body style silverado.  I am using the stock head unit with onstar, no bose. To intergrate my ihf1000 I used the cell site GMCSS-3.  Problem is I am getting engine noise though the speakers and I cannot figure out why.  I re routed the harness every which way, currently it's strung out across the seat and it still has the engine noise.  If I un hook the audio out from the ihf1000 and hook up the little speaker that came with the kit from motorola the noise goes away.  I have a feeling that the engine noise is coming from the wiring harness or from the stock stereo.  Anyone else have this problem?