Well, get the foundations/mountings checked.
Check also the same sound, when you reverse from stop. Release clutch same way u do when going fwd.
MAYBE.
Also, check under ur seats for some loose objects lying that may move and cause such sounds or smething in the trunk/boot...one time i had a bottle in trunk that created vry soft sound of tick evrytime i started from zero or brake
...it was so annoying...i found tht bottle and literally threw it hard...