"If isolating the cable modem with a transformer would solve the problem then I would consider it."
Does the cable modem have a 3-prong plug? Most do not. I don't recall seeing one with a 3-prong plug in a long time.
However, it can still be grounded through the cable TV line and there are ground isolators that can be used to solve the problem... The Viewsonics VSIS-EU is one such device. I read a report that a user on Cox cable got this supplied for free from Cox due to a ground loop problem involving a plasma TV.
There another place that it could be causing a ground loop--the Ethernet cable. Some cable modems were or are supplied with a screened twisted pair network cable. This has a metal shield around the RJ45 connector. If you are using a cable like this deposit it in the nearest trash can and replace it with a normal one.
Finally, if you are using the USB port on the cable modem I suggest switching to the Ethernet port. The USB port is a definite source of a ground loop.
Twisted pair Ethernet is, unless it's that screened twisted pair variety, isolated and will not cause a ground loop.
