I discovered in the Transformers FAQ that the fanfiction writers and newsgroup regulars have long debated exactly how the transforming sound is supposed to be spelled. At least half a dozen alternatives were listed, but none of them really "work" as onomatopoeias.
My own choice is something like: chick-chack-check-chock-chuck (or chuck-chock-check-chack-chick if they're going from vehicle to robot, of course). It sounds mostly like the cartoon/movie sound effect even if you just speak it aloud.
For the anal retentive fan, however, the Transformers FAQ has the technical details on how to recreate it:
The Transform sound begins as an 8 hz pulse repeated five times for 0.7 seconds each repeat whose pitch can be represented by the absolute value of a declining sine wave, with each zero-to-zero arc of the wave representing one of five cycles of the repetition. The actual value of the sine wave can be modified by factor N which is multiplied into the X value but divided into the Y value, which means that although the actual pitch is largely indefinite, in analog terms when the transform pitch is higher the length of each pulse is shorter, and conversely when the pitch is lower the length of each pulse is longer.
See also Transform scratch.