I just started my bike after not running it for a day. it does the knocking noise it for about 2 seconds when i first start it. then it stops and dont here of it again for a couple of days.i hope it isn't anything to worry about.

more info needed
what bike (i presume since it's in the fzr section it's an fzr)
what year model etc.
is it the engine, top or bottom end, starter, water pump etc
whip off the tank, start her up, see if you can locate the knocking,
post more info

its the 3tj 1994 uk model. havent a clue where its coming from and its only done it twice in the last month so dont really know where its coming from.

It's not much to go on, it could be something loose that gets shaken a bit on start up or something else.

You check the oil regularly ???

yeh ive checked the oil every time ive herd it and before and the oil level is fine. the bikes running like shit at the moment anyway as the carbs are blocked. so it may just be running on 3 cylinders for a bit.

if you think it might be running on 3, just start it up on tick-over and feel the downpipes, all 4 should be hot if you got a cool one then that's your problem,
probably carbies need a clean, slow running jet especially but clean the lot whilst you got em off, but could be coils or plugs
best bet is give her a good service plugs, filters, liquids etc

just re-read your first post again
"it does the knocking noise it for about 2 seconds when i first start it"
could be the starter clutch sticking, small casing right hand side, might be worth checking

Seeing as the tank had rust in it I'd say it's the carbs blocked up, change the fuel filter too

My dad and me are going to work in the bike over next week cleaning the carbs and the rest. it had oil change and new spark plugs only 600 miles ago. and a new casing which you are talking about after it fell over and broke it. cheers for the advice anyway.

are you sure there was no other damage done when it was dropped,
if you had to replace the starter clutch casing, did you check to make sure nothing else had been thumped behind it'
even blocked carbs wont make the engine knock (or running on three for that matter)

also as a side note,
if your tank is rusted up inside the best way to clean it is with a metal treatment acid you can get for about £8 for 5 litres from B&Q, brilliant stuff just take out the fuel tap and fuel sender blank over the holes, i used a couple of pieces of thin metal plate and a piece of rubber to act as a gasket, screw on with the bolts from the sender and tap and mix up the acid about 3 to 1 with hot water, pour in the tank and shake about every couple hours, then leave over night, drain and rinse with plenty of cold water and dry with the missus hair dryer, if your not going to fill it strait up with petrol then use about a cupful of petrol with 2/stroke oil in it, give it a good shake then drain and the tank will have a nice coating of oil to stop surface rust coming back and once filled with petrol the amount of oil will be so diluted it wont matter

we checked over the bike really carefully before putting the casing on. the casing was put on about 600 miles ago and its only made the knocking noise twice now. havent herd it since i posted so hopefully nothing terminal.
and ive already brought a new tank. as my current one was rusted so badly it almost had a hole in the bottom.
cheers anyway.