My 1999 XJ sport v8 auto has a strange single donking noise coming from my dash, it does not happen all the time, you can drive all day and it doesnt do it but then it will start again, any help would be gladly accepted.

 

Does it go away when you turn the hvac system off?

 

Went out this morning kept heater off heard nothing, then switch heater on still nothing,then switch off and within 10 secs then donk, but can also do it alot or nothing at all, sounds like its just behind dash.