Possibly parasitic oscillation. Do you have any photos of the chassis you can post? Be careful with squeal, it could possible damage the speaker and if its around 10k, your hearing. Try the chop stick method of trouble shooting: Take a chop stick (or similar) and poke about the circuit while the amp is exhibiting the problem. Move the wires a bit and see if it changes the sound. Parasitic oscillation can cause squeals, motor-boating and even no sound at all (even though the amp is cranking).

Be sure to keep the leads as short as possible in the amp.
Sometimes removing only a short length of wire will fix the issue.
Parasitics are not usually a problem with the Two Stroke if built close to DH’s original chassis layout but its possible. Confirm that the cap polarities are correct.