asterisk users - Mar 2005 - Echoing Beep on ZAP channel (one sided)

I have an asterisk server (Asterisk 1.0.3) which seems to be working 
pretty well with the exception of a terrible echoing beep on the ZAP 
analog channel.  This beep which seems to echo for several seconds at a 
time can only be heard on IP phones we use in house (we have no internal 
analog lines other than a grandstream ATA, just the one external POTS 
line). 

We have the X100P line card (2 although we only use one but have tried 
both for troubleshooting).  It only happens on calls that terminate on 
the ZAP interface, IAX calls and internal SIP to SIP calls have no 
problems.  The problem seems to be consistent on both Cisco and 
Grandstream hardware.  The problem seems to be especially bad when the 
voice mail system is taking messages.

Any ideas?

Thanks in advance for any help/ideas/suggestions.

--Andrew Duey