Switching power supply noise?

[jlo]

I have modded my Voodoo Labs Micro Vibe to run at 18v using the Dunlop  ECB04 18v adapter.   If my intensity knob is up all the way, and sweep the speed knob there will be a spot where there is a high frequency noise that cycles on and off with the LFO.  This happens even with the pedal bypassed or the guitar volume is turned off.   When I tried the Fulltone 'noise free' 18v power supply the noise is completely gone.   They are both 500mA.  What makes a switch mode power supply quieter?  Is it the switching frequency?  Additional filtering?   Why would the noise be heard in bypass?   I know that its not true bypass but uses an optocoupler...