What type? I using 9.5mm one, starting got airplane sound, after long time of troubleshooting, found out that is my HU RCA pre-out(rear)is the culprit. Actually, airplane sound got alot of cause, where mine is part of it. So if want o use a thicker one(10.5mm), I think you have to add grounding cables on your engine body.
Ok, back to the topic, your HU only have 1 pre-out. So your setup is something like this(70% correct, hopefully)
HU>pre-amp>front out>4 ch amp(front)>speakers, or tweeters
>rear out >4 ch amp(rear) >speakers
>sub out >2 ch amp >sub
If based on the above setup, is hard for me to believe that your HU got problem. If your HU really have this problem, then the left rear speaker or tweeter also will have the airplane sound. But so far you said only come from your front left speakers.
You have to bypass your pre-amp to test it already.