Turntable Hummmm

Suddenly, without anything being touched on my system, I have a hum coming out of the speakers when the stylus hits the vinyl. Its not real loud but it is there. I've disconnected everything and reconnected and it is still there. Any ideas how to find the culprit?
What cartridge are you using? Does it only do it when the stylus is in contact with the record, or is the hum there when the stylus is over the record?
I am using a Rega P25 with an Exact 2 cart. The cart is 6 weeks old and has sounded great. The P25 does not have a ground.

The hum occurs only when the stylus is in contact with the LP.

What is a ground lift plug? Thanks for the input.
if the hum only present when the stylus is on the record, this doesn't sound like ground loop, so I wouldn't worry about lifting the ground.

You say you disconnected and reconnected everything - does this include the signal tags to the cartridge connection? If not you may want to try that and apply some deoxit while you're at it.

If that doesn't do it, you may want to see if your cartridge is going bad. Do you have another cartridge to swap out to see if the hum is also there with another cartridge?
Did not disconnect the cart tags but will now. And I can swap carts if that does not work. Bummer.
When the stylus hits the vinyl?
My wild guess is the cartridge cantilever is bent or cracked and vibrating. Why not remove it and take it to your local, friendly audio shop? If you have another cart, try that one for comparison.
Good luck. Keep us posted.
Problem solved! Disconnected the 4 cart tags. One of them was not very tight so I took M. Fremer's tip and placed a toothpick in the tag and squeezed it with needle nose pliers. This tightened the connection. Hooked it all back up and the hum is gone. Thanks for your help on this irritating problem.