I've got a problem I haven't been able to sort out experimentally and am looking for help from someone who knows more than me. I'm making a handheld prop with audio and lighting effects. In the past I've used a commercial handheld amp (Aker VoiceBooster) as a base, tapping switched battery power from its circuitboard. That tapped power will run the controls, audio generator, LEDs, etc., and I'll use the amplifier on the Aker for the audio output. But this time I have a problem with an LED circuit generating noise on the power feed that is messing up the audio on the Aker. The problem seems to be that I'm using an ATTiny85 and pulse width modulation to drive a number of LEDs (with an N-Channel MOSFET because they need more power than the ATTiny can source). Simple circuit show in pic, LEDs on the right, noise apparently running back to the Aker's battery on the left. How can I fix this? I think the answer should be bypass capacitors, but breadboarding a number of different sizes across the 5V lines near the LEDs doesn't seem to change anything. (Tried 0.1microF - 10microF MLCC.) I don't have a scope to put on this, so I don't really know the nature of the noise (beyond its audible effects). I may well be missing something basic, this is just a hobby thing and I'm a software guy. Can anyone clue me in?
EDIT: The PWM frequency is the default for ATTiny85, which seems to be 500 or 1000 Hz. I was trying to avoid having to figure out how to change it, but that might be the best approach. There are actually 6 LEDs (I only drew 3), pulling 20mA each. The resistors are 100 Ohm. I've tried caps across the battery, and across Q1 source to the hot side of the LEDs, without much luck. But I'm not certain of the PWM frequency, or exactly what values/types of cap is likely to be effective.