These are the default FSX key commands for the VOR 1, VOR 2, DME, and ADF Idents that could be causing the continuous beeping:

VOR 1 Ident (on/off) - CTRL+1

VOR 2 Ident (on/off) - CTRL+2

DME Ident (on/off) - CTRL+4

ADF Ident (on/off) - CTRL+5

If you have changed them, look in the list of commands in the Kneeboard, under RADIO COMMANDS for what they have been changed to. You will need to give each one a bit of time to find if it's the one causing the problem.

If you have an Ident turned on in your default flight, you might want to change it to OFF, and then save it again. With some 3rd party addon aircraft, this is probably the best thing to do.