Hi,

I sincerely doubt an 80 watt subwoofer would effect the alternator.

Even though I will say, I have torn into some junkyard cars and found several feet of No.4 gauge copper wire laying about. Interesting clear to pink insulation so you could see inside to the copper strands. It’s all Show stuff!
It can carry some serious amperage or wattage if the voltage was higher than 12 Volts. I have trunks filled with batteries.
Staying down low in voltage costs lots in wire to get the same work done. I hope your 80 watts is install with rational sense as it’s peanuts to your alternator. What size is the fuse that powers everything?

The hum of a well tune tractor is a new description for me.
In Sweden, the Volvo people there have nicknamed them “Tractors” over the many of years past! So you maybe on too something?
“Bricks” is a, relevantly, new modern term.

Tractors, of the diesel variety here in the USA, have a deep sounding grumble?
A slight low sounding rumble could be a bad rubber joint inside crankshaft vibration dampener. Put a line of paint across the front two halves and see if it shifts the line apart later on.


If you hearing the sound from the alternator, cause by your audio system, if you turned it off the hum should go away. Logically!
If the audio is ON maybe it’s a loose windshield or inner door panel? (-:) Just Kidding!

The high idle should not happen as the fuel management system all ways adjust for additional loads and is designed to hold 750 rpm and only slightly higher is allowed by the ECU when it has been told the A/C is on.

A higher idle is usually an air leak beyond the control of the computer.
You can suspect a bad brake booster or a check valve making a hum if it is passing lots of air it can vibrate the reed at certain flow rates but that on the other side of the car?
The cabin vacuum bottle has one is on top of the engine.
Both check valves are not going to be very loud though.

About the only noise than can be made by an alternator are the ball bearings going dry or at their end of life.
The alternator can make a squeal if it is excessive overloaded or when and if the belts slips due to that.
A bad cell in an old battery can yank a load on the alternator or a bare cable touching some place it shouldn’t. Under the front of the engine is place to inspect for some melted sheath.

Hope this helps you look and listen a little deeper.

Phil