UCC256302: Transformer making sound in UCC256302

Hello,

We have made prototype of 27V/20A SMPS using UCC256302.

In burst mode we are getting sound from transformer like..TIK   TIK   TIK   TIK . Repetative burst is at every 20ms.

After 0.44amp. burst mode ends and sound also disappears, thereafter upto 20amps. there is no noise from transformer.

Transformer details is as below,

core: PQ3535

Primary Turns: 24

Secondary Turns : 3+3

Auxilary secondary Turns : 2

Lm: 400uH and

Lr: 9uH

External series inductor : 30uH.

What can be the reason for this sound??

Any solution??

Thanks and Regards,

Asif.