when you press accelerator it knocks any one know why

Usually knocking means the gas that was put in is poor quality.

The guy who is selling should have put premium in to prevent you from hearing that! Is the vehicle supposed to use premium and they've just been using regular?

the accelerator is knocking because green ooze from the engine has leaked into the spedometer causing it to knock

Possibly bad gas causing incomplete combustion as Tim has suggested but l would not think so.

l would investigate this knock further as it could also be worn out bearings or maybe just a lifter tapping & best case a bearing going bad on a belt tensioner or... l would try to pinpoint this if u feel it is internal by using a dowel (wooden rod) or long screwdriver - place one end onto ur earlobe & the other on the block while u have someone rev the motor to cause the knock. Doing this greatly amplifies the sound & by moving the end around to different parts of the motor can isolate & narrow down where it is coming from. Noise from the top end can usually be fixed without a major expense, bottom end usually expensive. This may not be a vehicle worth purchasing.

If uncomfortable trying to find it yourself, bring it to a trusting mechanic & ask his opinion.

Good Luck

WAIT..

There is a purge solenoid on the 99-04 JGC that could be what u hear. Drivers side of the engine compartment, behind the air cleaner, attached to the body itself. Its a little black round valve that ticks & sounds like an engine tick, but isnt. Put ur hand on it with the motor running to see if this is it. Its completely normal for this to make a tickings sound - there was a technical service bulletin that fixes it, just by moving it away from the body.

If its cold this is more likely to happen

It could be a bad bearing.