Squeak when shutting door/ bonnet

[WeaversT6]

As above, when I slam any of the doors or close the bonnet there is a squeak from somewhere ‘within’ the chassis/ engine etc. any ideas on what to check?

Wondered if it could be suspension related??

More of an annoyance than anything else.

 

[Paynewright]

Can you make it happen by rocking the van slightly? If not could be one of the air vents in the rear quarters low level. The have rubber flaps to let air out but not in (either from heater unit inbound or door slamming). Perhaps acting like a reed (music instrument stylee)