Noise during acceleration

I have a '91 LX and I am getting a 'fluttering' noise during acceleration. It stops when I'm coasting or idling. Any hints as to what I should be looking for?

Also, an update on intermittant stalling - replaced O2 sensor and it hasn't happened since, but it was only occuring every 3 weeks or so, so I'm not convinced it's resolved yet.

Thanks!

Hmmmm... is it auto or manual? My Mazda sometimes makes a sound when the air filter hits against the hood of the car. Maybe that's it.

Thanks for the response, as for your question - I have a manual transmission. I'm thinking it's not the air filter because that's buried underneath what I think is the MAF sensor. Anyway, I know it's under something.