A popping noise when comming off idle

[Kiwi]

This is hard to explain... but here goes.
If i am cruising along, say in 5th.  And when i let off the throttle, basically to maintain speed not increase it.
I hear a popping sound under the gas tank.  I think it's coming from the rear cylinder, but i could be completely wrong.
When accelerating, all sounds good ( sound really nice actually )
It does seem to happen around 2500 rpm, but it could be a touch higher or lower.
The carbs are somewhat synced... I need to get the proper tool to do it well.

So... Basically.. when I ease off the throttle, i often get a popping sound from the carbs.

What adjustment is needed?

[Tiger]

 :) Check that the two clamps that hold the exhaust "Y" pipe in to the rear cylinder head are tight...you may have an exhaust leak which, on deceleration, can give this symptom :-\

              8).......TIGER....... 8)

[Night Vision]

popping on decel can indicate a lean situation... vaccum leaks or lean mixture... you got staintunes right? might have to rejet

[Walt_M.]

Could also be the exhaust valves are a little tight. You don't need the tool to check the clearance, it just makes it easier to change the shims.

[Kiwi]

I found a small leak from the front headers, plugged that up and the noise went away. 
Thanks !