I've noticed a whistling sound that seems to be coming from the end of my exhaust. Happens when idling and while driving constantly. Could it be my cone air filter ive fitted?

could be, but a whisting exhaust is more likely to be a whistling exhaust

Any ideas whats causing it? Only seems to be since the cold weather in the morning. Leaking exhaust? I had a new exhaust fitted (back Half) and they had trouble getting the old on off by the cat so could it be a bad fitting?