Hi all,
I just replaced the speakers in my miata with some nice new ones in an effort to fix the terribly sound quality and hopefully eliminate the really annoying buzz/hum present at all time. The sound quality is much better now. Not a surprise as most of the surround and part of the cones on the factory ones was missing (see photo). However, that annoying noise is still there and seems louder. It doesn't change with RPM so I don't think its ignition feedback. Where should I start with troubleshooting? Its a 99 with an aftermarket JVC deck and brand new Rockford fosgate speakers. The car did have the Bose system but I didn't notice the amp when I reinstalled the stereo to fix the PO's shoddy work. The sound existed before and after I fixed the stereo's installation.
Old speaker (better of the two): 
In reply to aircooled :
1.) Thanks! Sounds like I should pull the stereo and check if any grounds share the same bolt, is that right?
2.) Ruh roh (from the article): 
Actually, you WANT them to have the same ground. The issue appears when there are multiple paths to ground that have different resistances.
https://www.crutchfield.com/S-R5pLRjxh0CS/learn/learningcenter/car/noise_suppressors_installation_guide.html
https://www.lifewire.com/whining-noise-when-i-use-aux-input-534592
The Crutchfied link talks about other possible causes also.
Very strange they would put that note about the author in the article...
There's either an amp hiding somewhere, the head unit is bad, or some really bad grounds and something else that noisy. Try hooking a speaker up directly to the head unit output and see if it buzzes.
In reply to Dr. Hess :
I'll check again for the factory amp while I'm in there checking grounds.
parker
Reader
5/9/19 4:58 p.m.
I'm pretty sure I have ignition feedback. How do I address that? 1998 Toyota 4Runner with newish OEM plug wires.
Well, my idea is gone then. I was going to ask if you ran the coax cables for the amp next to the power cables but that's clearly not the case now 
Hopefully someone with more knowledge will chime in for you.
For your ignition noise, I'd try a filter on the +12v line going to your head unit, as close to the head unit as you can get. If you have a distributor on that motor, try a new cap.
I'm not sure I am clear if the hum is present when the key is on Acc - so without the engine running. If so, that opens another whole can of worms.
I have it in my truck with the factory radio. It is a motor in the vehicle. I suspect the fan. I get the buzzing with the ignition on but the motor not running. Turn off the ignition and it is as if a motor/fan is spinning down. I just need to find it.
OP: Sounds like a harness short. Is the aftermarket harness (radio to stock harness) properly soldered or using those stupid berkeleying butt connectors?
Parker: You should make sure it's not the alternator causing the noise before taking drastic measures towards the ignition system. Also just to be safe ensure the ground is secure and there are no shorts in the harness. A radio power filter would be a good bet. Capacitors have been known to help if you want to go a more homebrew route, but a power filter should be relatively cheap.
My last stereo had a short in the harness. I just smacked it and it would stop humming.
First culprit I go to for a buzz that stays constant (doesn't change with RPM) is the fuel pump.
I had one GM that the air injection pump was making a little bit of noise, but that was primarily my fault. When I installed the fuse and big wire for the amp, I didn't realize I was putting it right beside the air pump. A slight re-routing of the wires fixed it.
Find out where to hot-wire the pump. Most Mazdas have a pigtail somewhere that you can jump a wire to the battery to test it. Key on, the ECM should kick on the pump for a few seconds to charge the rail, then come back on when it gets crank signal. If you sit in the car KOEO and have someone cycle the fuel pump on and off, I'll betcha you get a hum, then no hum.
It could be a noisy (electronically speaking) pump, or you could have a ground that is becoming insufficient. This problem is usually exaggerated if you changed pumps... either a good quality high-flow pump (since it tends to draw more amperage) or a stock replacement from the parts store (since it was made by the lowest bidder).
Also try a bare wire from the chassis of the radio to a ground on the frame or dash to see if it changes. More hum or no change can mean noise in the power supply side. Less hum can mean you need a better ground, but those are very vague diagnoses. Electronic noise is a wonky thing.
In reply to N Sperlo :
Unsure. But I assume the bad way, based on what I've seen so far.
= :)
= :(
Thought I had a better pic, but that was the best i could do. Ignore the ash and soda scum. I cleaned it up.
