The NSR150SP can be a noisy little bugger. Ive done a couple of noisy 150's and it was down to piston/bore clearance. Hunted through pistons and got the clearance at 0.04mm. This seemed to work. Not perfect, but better.

I even know of one guy that complained to AP Honda (Honda Thailand) about a noisy 150SP and he was told, they all sound like that mister.

You really have to consider the price and realise that these are 'made in Thailand' budget bikes. I know that you want something to purr but that ain't gonna happen with an NSR150SP!