As usual, this is thoroughly documented to a perhaps ridiculous degree on the Transformers Wiki:

https://tfwiki.net/wiki/Transformation#Onomatopoeia

There is an official answer to this question. It’s Transformers; of course there is an official answer to this particular question. Actually, there are several. Jury’s out on which of these interpretations in particular have caused the franchise to be Ruined Forever, but surely at least one of them has.

Optimum Pride

Jersey-Jersey-Shore

I’m sorry, but this belongs in Stupid Questions

Hmmmmmmmmm

Does not matter if powerplant transformers or the one in your phone charger. Same Sound.

ER-AH-OU-AH-ER

I don’t know, but this is an awesome question

transforming noises

aaron earned an iron urn

DJRRT- DJRRT- DJRRT- DJRRT- DJRRT!!

Wonder if there’s a book/novelization, since that would be canon.

ki-kiri-ki

Well, there’s five beats, so…

Ts-che-chu-chu-chk

(autobot transforming sounds)

Khou-chk-chk-chk-chk