Don't believe there is a "donut" on the exhaust system for the Focus. Donuts
were used on the old style exhaust systems which had a bell joint.
The Focus exhaust system is a follows: the manifold is connected directly to
the catalytic converter which is connected to a flex pipe, which is
connected to the front muffler, which is connected to the rear muffler..The
flex pipe allows movement of the exhaust system as the engine moves.
It could be an engine mount, the way to check it: block of wood on top of a
floor jack and when it is making the noise slight jack up the engine (about
1 inch). If the noise stops its one of the engine mounts or the transmission
mount that is bad, if the noise does not stop its something else. The only
way to check and engine mount is when the weight is taken up on it.
Failing motor mount. It gets worse, and is first noticed in the cold when
the rubber is stiff, but as it gets looser is now noticeable all the time.
The vibration has now damaged your exhaust system, which causes the noise.
If you're lucky it could only be the donut (which separates the manifold
from the exhaust pipe). The sooner you get it into a dealer or good repair
shop (one able to do engine work), the better. It will simply cause more and
more damage if ignored.