Hello, I hope the following is of assistance.
First you need to confirm if it is the vehicle horn that is sounding, if it is a constant tone as though the horn is being pressed then try disconnecting the horn. Remove the horn fuse, this is in the fuse box under the bonnet on the passenger side, it should be the 4th fuse up on the left and is shown as F1.7 in the diagram below.
If this stops the noise then remove the horn relay, this is in the same fuse box and is the left hand relay of the line of 4 relays that is marked K36 and C1045 above, with this out refit the fuse and test. If the horn isn't sounding then check the relay and also make sure the horn button isn't stuck down on the steering wheel.
If the noise is the same as the horn but goes on and off then it may be the alarm, try disconnecting the security module, this is behind the dash on the passenger side and has a blue and green connector to it, disconnect these and see if this stops the noise.
Start with these points, let me know if I can assist further
r and I'll reply as soon as I'm able to.