dist pot feedback

[ptrx]

hi,
i just finished my first project succesfully and without too much hassle. it's a dist + made from recycled and new bits and it works great, thanks to everyone on this forum and other sites you've kinda made this pedal for me!
one thing though: when i turn the dist pot all the way up, i get a high pitch tone. how come?? it's no big deal i just leave the pot a bit lower but i'd still like to know the reason for this..

cheers
pat