I am in the process of writing a review of the Sumo R/C line of 1/24 scale vehicles. I have 3 of them that I have been testing for about 40 days now (with the help of my 7 year old son). Without intentionally breaking the trucks, I am attempting to tear them up with hard core use including large jumps landing on cement, running 7.4 volt lipo batteries and running in dusty/dirty conditions.
Now that my preamble is out of the way – all three of my trucks make the sound you are describing. I have not investigated the origin, but I will. I am not sure if it is from the speed control, motor or steering servo. My guess is that it is coming from the steering servo. I have had other servos make the same sound on trucks, helicopters and planes and it is quite normal.
Have fun with your Sumo – and you may want to look into some of the upgrades like the faster motor – with good batteries and the motor upgrade you can do wheelies.