Getting worse MPGs and hearing a knocking sound???

[dyldorichardson]

Recently changed the air filter on my G35. It seems ever since I changed it I have been getting worse and worse average MPG. I haven't changed the manner in which I drive. Not sure if the two are related but it's an odd coincidence...

More importantly, I just noticed a knocking sound when my car is in gear. If I have it in neutral the knocking stops, but as soon as I put it in drive it begins again. I thought I might need a new serpentine belt but I opened the hood and it didnt sound like the knocking was coming from the engine. Thinking something in my transmission may be causing the noise???

Also, ever since I have owned my G it makes a very odd noise when I first begin driving on a cold engine. It sounds like a gear slipping or something, but it only occurs in the morning when I first start my car, put it into gear, and start driving. The sounds never occurs twice in one day, its only when the engine is cold and it hasn't been driven in several hours.

It is a 2003 G35 Sport coupe with an auto transmission. Has just under 109,000. The last tune up I had done to it was at 90K. I am currently trying to sell it and don't want to have to take it into the shop again to have it checked out. There are no indicator lights on, and the fluid levels and everything appear normal. Have any of you ever heard/experienced these occurrences before? Any advice/help is greatly appreciated. Thanks guys!

[telcoman]

I think the simplest thing would be to put your vehicle on a lift with the engine running and put it in gear while someone observes underneath, Could be something as simple as a broken exhaust bracket or a catalytic converter issue?

Telcoman

[joe603]

Well you may have damaged the throttle body or the MAF sensor. Did you clean either during the filter change? Might explain some of your issues.

[dyldorichardson]

Thanks for the ideas guys.I have uploaded a youtube video that shows a loud rattling sound from the rear area of my car. I don't think this has to do with my previous post but any ideas? It's made this sound for a while now but appears to be getting worse.

Link: http://www.youtube.com/watch?v=IjNMGlOYOtg