Every line inside the D180 that can produce noise has an EMI filter on it. We had issues a long time ago with the D10 and learned our lesson. We do lots of testing to prevent this, so your experience is pretty unique.
It tells us something that other people hear your transmission OK. It means that the noise is on the headset speaker lines, not the mic lines, so it's probably not RF interference. My first guess would be that this is a grounding problem somewhere, which is causing the ground of your phones to dip a little when the EFIS draws power. Since the EFIS doesn't draw perfect DC power, this can sound like noise if it gets into the phones.
I'd make sure your headphones are only grounded at the SL-30 or your intercom, and not at the airframe or to any other wire. I'd then make sure the D180 and the SL-30 have good, solid grounds and power lines to your battery without sharing any small wires along the way. I would probably run an extra ground to the case of the D180, especially if your panel is composite instead of metal. Composite panels don't ground all the cases together well so they can make some grounding issues much worse.
If your D180 is hooked to the SL30 via serial, probably worth disconnecting this wire as a test. Not something we have heard of as a problem, but you never know.
Let us know how these things go and we can work from there if they don't help.
