Constant hissing noise from speakers after aftermarket head unit install

A few weeks back I replaced the stock head unit in my 2009 Legacy 2.5i SE with a Sony XAV-AX100 head unit. Everything works great except for the fact that there is constantly a very subtle (but noticeable) hissing noise that I can hear whenever the head unit turns on, and it wasn't there before.

 

Do any of you know what exactly might be causing this? I have the Harman Kardon sound system, so the new head unit is connected to the stock H/K amp - could it have something to do with the amp?

 

Or perhaps it's a grounding issue? The Metra wiring harness I used had a black ground wire, so I cut off the ring terminal on the ground wire on the head unit's harness and crimped that (plus the parking brake wire) to the ground wire on the Metra harness. Is there something wrong with this method?

 

I'm quite new to car stuff in general so apologies if these questions are a bit dumb. Thanks!

Does the hiss increase with engine speed? If so, definitely a grounding issue.

It does not change with engine speed - it doesn't change with anything, actually. It doesn't change with the input source either. As long as the HU is on, it's there and it's constant (although very very subtle). Could it still be grounding?

Yes Grounding or loose connection.

 

Laughing at Oneself and with Others is Good for the Soul [emoji847]

I would avoid piggy backing on another ground. Wire a fresh one and see if it goes away.

Some head units can produce the hiss, maybe internal grounding? I know some Sony units can, I owned a couple that hissed and my dad's old car has a noticeable hiss with Sony HU, all running external (4ch) amps.

Not just limited to Sony either (IIRC Kenwood had some issues a few years back).

Thanks for the suggestions everyone. The ground from the HU is currently wired to the ground that comes out of the vehicle harness - I'll try wiring it to a chassis ground instead and hopefully that will clear up the issue.