Buzzing relays? Anyone help?

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After a successful install of my automated hot water/central heating
controls (with many thanks to Neil Fuller for his donation of a foot of
DIN rail to securely mount the relays!), I've just noticed a small but
annoying problem.

The relays buzz slightly when they are off. The installation is
basically two big 240V relays connected into the CH/HW wiring, the
relays have 240V coils that are switched by a pair of X10 appliance
modules due to the distance between the server and the airing cupboard.

At a guess I'd say that the leakage current from the modules is enough
to make coils buzz. The problem is exacerbated by the relays being on
one side of a plasterboard partition wall, which seems to amplify the
buzz into the next room. Aside from the buzz everything else seems to
have worked just fine for the last few weeks.

The $64K question is how to stop the buzz? Any ideas?

Lee