Noise on computer speakers - from test circuits

I have my test power supply hooked to "Gateway" generic computer desk speakers. It is just a 12v switched cheap-o 60 watt thing. My desk LEDS are also hooked to the power supply. Audio line of the computer speakers plugged into my desk PC sound card output.

I'm to building a 555 timer circuit, with IRF540 hooked to output with some 12v strip LEDS. For reasons unknown - the noise from the 555 timer mosfet comes through the computer speakers. I turn the potentiometer up and down and can hear the PWM signal through my speakers.

The output of 555 timer is connected to 10k resistor, to GATE pin of IRF540. S pin to ground. +12v to led strip+, led strip- goes to the drain pin. If I disconnect the 555 timer from the mosfet, the speaker noise goes away.

Is there something I should be adding to my circuit to prevent feedback through this shared power supply line?

Ideally I want this pwm circuit to DIM the LED strip I have connected to desk lamp, and the noise through the speakers is too distracting. I figure it's a sign I'm doing something wrong.

I just learned how to use my cheap lcd oscilloscope to see the pwm pulse signal. When I apply scope to the +12v line in my circuit, I can see the power line waving around a little bit - to the signal of the pwm. I do not know what I should do from here - what circuits I need to stabelize or clean that +12v supply into the device I'm trying to make. I can use some advise, please.

If I put a 220-uf capacitor on the DRAIN to GROUND, the noise on +12v will reduce, but does not go away.

Ok after all this time, it turns out the OSCILLOSCOPE was actually making noise. I disconnected by accident and noise goes away.