Listen, this is going to sound a daft question but - how should a 1s motor sound when it's running?

The reason i ask is because although mine fires up on the first kick, hot or cold, it's making a kind of rotary knocking sound up until about 5000 rpm then i can't hear it over the actual engine noise....

As i have'nt got another bike to compare it with, i don't know if it's shagged in some way or 'they all sound like that sir'??
It seems to rev okay and there is no excessive smoke coming from either exhaust but it's just this 'Diesel' type sound thats getting on my tits.
I have'nt even touched the motor since i've had it as i've only just got it back together after a year in my garage and i don't want to guess at what it might be as i know throwing money blindly at a bike does'nt go down well with my bank account....
I hope this does'nt come across as the stupidest question posted on here but i am a bit concerned as i don't want it going boom when i take it for it's maiden voyage!
If you have any ideas please leave me a message as imy laptop is screwed and im using someone elses computer and unfortunately can't answer questions as quickly as im used too....

Thanks Guys
Kris.