clicky sound after highway driving

Somewhere under the dash there is a relay that likes to click very rapidly after highway driving. Around town I never hear it although I can make it do it if I try to almost stall the engine.

Typically it does it when in a 110 zone and slowing (dropping out of gear) when comming up to an intersection. If the revs go up a bit it doesn't do it when at idle. Only does it for a minute. If slowing into a town to 80 then 60 it generally doesn't do it as it hasn't been a sudden de-acceleration.

It normally idles around where it's supposed to.

Does anyone have any suggestions why it might do it and what it means.

it's a 2.8td GU 99 patrol manual.

Cheers for any info people may have.