Hello Stephen @sarcher1228 and Chris @chris101,
My first encounter with noise from the synth speaker was about external noise signals that entered the speaker bit through the 5 Volt VCC connection, and it was caused by a servo, powered in the same circuit.
My main conclusion was that the synth speaker is extremely sensitive for noises...
But your circuit is something different...
I copied your circuit with 2 synth speakers and discovered a few interesting things.....
I also heard this funny squeeking noise when I connected both speakers to an amplifier via the output connectors from both synth speakers...
I connected speaker one to the left channel and speaker two to the right channel.
All GND connections connected together.
Disconnecting one speaker and this noise was gone indeed..
Next I hooked up my oscilloscope and did some tests.
This is the funny noise that should not be there:
Then I connected the amplifier directly to the oscillators via a proto bit.
Both GND connections to GND from the amplifier and the signals to the left and right channel.
A very clear sound without any noises.
Next I connected the amplifier via both synth speakers and saw the same signal, fully disturbed by high frequency noises:
The noises are fully caused by the synth speaker bits.
This is the circuit of the synth speaker bit:
On the output side you can see an amplifier IC type SSM 2305.
I think the output signal (out+ and out -) might be floating from the GND connection, so funny things will happen if both output - connections from both synth speakers will be connected via the amplifier.
I connected my oscilloscope between both "output -" connections and yes , there I could see the noise signal again...
Then I tried a few trics to eliminate the noise and only two things really helped:
Adding a 1 MegaOhm resistor in each signal connection from the synth speakers to the amplifier weakened the signal, but the noise was completely gone.......
Connecting the amplifier directly to the oscillators via a proto bit solved the issue also.
Hope this helps,