I have a 900cbr and it's got a light tapping sound. Replace 2 plugs and sound isn't as loud. Should I replace the other two and will that fix it. It's a 94 900rr cbr.

 

" slight tapping sound" wouldn't have me replacing 2 plugs but seeing as it's quieter why not change the other two whilst your at it, nothing to loose.
The tapping sound would have me checking the cam chain area or top end clearances. How is the bike generally running, sounding ? Plug colour and condition ?
The more information you can give, the more likely any issue can be resolved.
Best of luck mate.

 

I agree with wes.....

You should change all of the spark plugs not just 1 or 2 whenever you're going to do plugs. Look at quality of fuel, condition of the oil, etc. I think whenever you buy a used bike you should always change the oil, oil filter, fuel filter, plugs, etc. That way you have a good starting point for preventative maintenance in the future.