Help! Whining type noise that stops between 5 & 10mph

[habib]

OK turns out that nut I bought off ebay is made of soft metal and the threads have worn loose. I put the old nut on without the lock plate for now and that solved the issue. Ordered the normal nut and lock plate. So definitely if that forward pulley is loose by a few millimeters it's going to whine like banshee.

I had a chain conversion done a bit ago. Since I am running a sprocket is the stock locking plate still used?