'Ping' ?

hi all,

i've recently been suffering with a couple of fault codes,

car is a 2001 1.8t 150bhp L&K edition.

17705 - now fixed by replacing a hose

17536 - still an issue (i had hoped it was related to the 17705 but unfortunately not the case)

17536/P1128/004392 - Fuel Trim; Bank 1 (Mult): System too Lean

the above causes the engine light to come on and stay on until cleared. then around 100 miles later it will come back.

i've had these codes for a while now but as they made no difference to how the car drove i never hurried to get it looked at.

since last week when i accelerate then lift off to change gear the car makes what i can only describe as an electronic 'ping' sound. which sounds like it is coming from under the pasenger seat.

can anyone tell me what this sound is for? or what may be causing it? is it likely that this has occured due to the above error code?

thanks for the help,

Shaun.