Speech Therapy?

I think I have a speech problem (other than Weese Weathers). Every day I say the word "flash" more times than I can count (optometrist's office, you know). And almost every time the "sh" comes out with a sort of whistle. Now this is odd because I can't actually whistle on purpose. I sound like the beaver on Winnie the Pooh. I have no solution to my problem. Any advice?