Hello, I recently buy a 1990 gsxr 750, and the engine has got a noise as if a washer or piece of sheet metal vibrated. I don't know if is normal in those engines or not, because is my first gsxr.
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FYI, I think your engine sounds fine...they are mechanically noisy, as they don't have a water jacket to dampen the noise.
Check that your exhaust bolts are tight.
Synchronizing the carbs can reduce some of that noise as well.