2001 Neon might have knock

ok well im trying to sell my neon since I got another car, its just a 2001 se, base everything, auto blah blah.

it hasnt really ran in 3-4 weeks, and its about 150mi from needing an oil change, and someone comes to look at it and we start the car and I hear a noise, and it really sounds like a mild knock.

so im thinking awww **** I hope he doesnt notice because it sounds like knock, but he did. he even said sounds like a knock, drove a little, and it didnt go away, so he was like yea im not interested and turned around.


so this is a kick in the nuts because I dont know what to do about it. never had problems with the engine and got my oil done on time. im not to sure about non turbo cars, but generally you can do a oil change (since it needs one) and if that doesnt help maybe some higher octane gas?

need some help asap!

Also I noticed last time I had the oil changed (didnt care about this car so took to a shop) my gas milage was horrible ever since, i get the feeling they didnt replace the filter.

If the noise kind of a sucking noise then you may have to tighten up your spark plugs well thats what was wrong with mine.