Little pink on throttle openinh

[Jvr]

Hi guys 

A strange one for you

I've noticed when my 2.5 warms up when you apply throttle from no throttle or overrun it does what sounds like 2 very quick little pings

At no other times does this do it e.g.

When gas already open hot or cold,under load and either octane fuel makes no difference only as in first paragraph

Any ideas?

Cheers

P