What is causing this?

[rousejeremy]

http://www.youtube.com/watch?v=k84hvLGZZEU&NR=1

[GibsonGM]

COOL!  Very nice paint job, by the way!
You could scrap the fuzz, mount it in your car, and there's your "cucaracha" horn!

Ok, down to business.   Oscillation is caused by a regenerative feedback issue.  Which is where some of the output of an amplified stage is fed back to an input, causing the output to go higher, which is fed back, etc etc etc. It must be IN PHASE with the signal it is reacting with, so it's not coming right after an inverting stage.   Sounds like something capacitive, too, because it is changing pitch like that....a capacitor is discharging and recharging, changing the frequency it's resonating at.  The signal could be being fed back from a wire (gain/vol pots are frequently culprits); could be a bad connection or miswired connection (did it always do that?), possibly an issue with the power supply not being decoupled.  This happens more with a high gain circuit like distortion/fuzz.  Does it go away when you play, and then come back as the note fades, or is it constant?

Cures:  open her up, and try to get it oscillating.  Move wires around gently with a wooden stick (pencil etc).  See if that changes things; if so, note where it changes.  Sometimes if you chopstick like that, you can fix the lead dress and get it to stop. In which case you must tie the wires into the right position.  This is assuming it is built correctly, which you might need to review before diving in.  A building error can wreak all kinds of havoc!
If you find it's wiring to a pot doing it, you can use shielded wire to/from the pot, grounded at 1 end only to stop that capacitive coupling.  Same with the input jack.

Another idea....how is your power supply decoupled (filtered)?  Do you have a schem. you can post of just how you built it?  Adding some R's and C's to clean up the power supply can sometimes help, as can bypass capacitors.   Really need a schematic to look at!

Hope that helps you out a little, post back with some more info! 

~Mike     

[rousejeremy]

Can you believe that sound was intentional? It's a Fuzz Factory clone in oscillation.

The Capacitance thing you mentioned may be it. The feedback sound bends pitch wise when a note is played. I thought it was really cool, just never understood how it was happening. Thanks for the info!

[GibsonGM]

Sure! I thought it was just doing that all the time, ha ha.  Self-oscillation...can be used musically like Hendrix did it, or it can run out of control!  Sometimes the wiring is 'just so' and that happens...