Your design is too dense or your needle is dull. Does the PE 770 come with any built in designs? If it does, run it through one of them and see if the problem disappears. If it does, it is the design. If it doesn't and you have changed the needle then something else is going on and it may be time for a service call.
If your machine has no built in designs use a design you know worked fine before the noise started.