roadie3rd

Hope someone can assist
I have a 2001 1.6 ford focus elle,
The problem im getting is that when the car is in idling it sounds like a deep echoey popping noise from under the bonnet. It also sometimes feels as though its about to stall when pulling away from the lights or a junction and in some cases (not so often) get a loud pop (almost like a back fire) when engine is struggling. Could it be to do with the cone air filter I fitted

When doing motorway driving its fine or in high revs its just when you are doing slow round town driving it seems to be doing all of the above

Its diving me insane

Please help
Mark