Has anyone run accross this on a BCM 50 RLS 2 with PRI

When we do a conference we get a loud squeal.
it's ok when the first 2 people are talking but the moment you conference in the next person, all that is heard is a squeal. It doesn't matter which person tou let go from the call, it clears up. Conference them back in and it squeals again.
This happens with internal and external conferencing.
Have changed the DTM module and made sure the patches are up to date.