Bird chirping noise on start up - GQ EFI

[NissanGQ4.2]

Hi all

My old mans 91 EFI GQ Patrol has developed a sound like birds chirping on start which goes for about 5-10 seconds then disappears, doesn't do it on every start either, sounds like its coming from the distributor but could be coming from elsewhere.

Any thoughts?

Pretty sure he has removed / replaced belts with same issue, so not the belts

Cheers

Toddie