Topic: ff400: weird cracks & pops & echo

my system:
ff400, macbook pro 13inch 4gb ram. os x 10.6.4
drivers: I?ve had a previous version installed & just installed the new version 2.87 (including flash update).
I also have the fireface usb drivers installed.

my problem:
there are random cracks & pops when sending a signal to input 4 on the fireface400. when these happen, the SIG leds of inputs 1, 2 & 3 light up. the cracks & pops don?t happen at other inputs.
also it seems to me that when this happens I hear a echo of the signal for a second or so.

any idea?