Nissan Repair: Nissan Sani V6 3.0L 98, clutch pedal, 3000rpm

Question
Hi there....just referring to the question i posted earlier re hard starting...i just want add that once the engine starts it goes perfectly...no problems at all. I tow a caravan often and have also had a "chip" installed to improve performance etc. I have ONE more question to ask you....
When i am towing the caravan (engine under load)...at plus/minus 3000rpm in 3rd gear a "ttrrrrrrrrrrr" noise is heard inside the car to the left of the clutch pedal or somewhere under the dash in that vicinity. Almost sounds like a relay or something??? If i lift my foot from the accellerator pedal or change to another gear the noise goes away. It does not do this all the time, but i am worried in case it causes a major problem.The engine timing is 100% so do not think it could be a "ping". I would like to know what electronic component could cause this noise??

Please help!!

Thank you>

Answer
Darryl,
Sounds like a ping or something reaching its resonant frequency with that certain load.  Try octain boost and see if it goes away.  Also, inspect the firewall, both sides, to see if anything needs to be tightened.  There are relays and modules on the firewall, inside, under the dash.