Buzzing on deceleration or during gear changes

So I have this weird annoying buzz when I lift off the accelerator. It happens at about 2500rpm.

Thought it might be a loose push rod cover tin but I checked them and they are all tight.

I have put on new heat exchangers so could it be a bad seal?

 

Is it coming up the gearstick? May be the bottom of the gear lever vibrating.

 

Nope can hear it through the heater vents as opposed to feeling it

 

Something caught in the cooling fan?

 

Nope checked, it is only around that particular rpm, maybe I should do a video?

 

Rattley tinware?

 

thermostat flaps?

 

Well I did think that but I tightened the lower tins. Maybe I will take then off and see if it makes a difference