GGG Bluesbeaker Question

[wavey63]

 I built a Marshal Bluesbreaker clone from the GGG site and although its good, I have an annoying issue with it. With the gain at about 2 o'clock I get a weird trebley "splatter" for lack of a beter term. It almost sounds like a blown speaker might. I took out the clipping diodes and also checked and rechecked all component values and it all seems ok. Any advice on how to tame that shrill noise?