Whistle coming through speakers

Alternator noise most likely. Does it shift frequency as rpm changes? If so then it is feedback from the alternator. It could be through the wiring or it could be through the antenna. If it is something that just started then you will want to have the alternator itself tested for AC ripple as that is the common cause for the whine.
The way to test if it is coming in through the antenna is to remove the radio itself and unplug the antenna cable.
Is this the factory radio?

If you find it is the antenna the common issue is the outer wire shield has lost connection to a good ground. One thing that I have seen work for this is to use aluminum duct tape to wrap the antenna lead then make sure it is grounded. Or you can replace the antenna lead. (Or if you do not listen to AM/FM, just leave it disconnected).