If the car is in 5th gear at a low speed, say 30 mph, the engine makes a rough rumbling noise, almost as if there is a fuel delivery problem or the pistons are missing.

After replacing the spark plugs, the car ran much better for about a day. Then the low rumbling noise came back the same as before.

The next day, the low rumbling noise was worse than before ever changing the spark plugs in the first place.

I am now thinking that I will give each spark plug one last little tug just to make sure they are all on tight enough. (Although they are probably just fine.) Also I certainly will not put too much force into retightening them for fear of messing something up. Then after that I might replace the fuel filter, since I have heard this is not too hard and fuel filters can cause problems and hey why not replace the thing anyway.

What does everybody think? My next step is to look for an automechanics newsgroup, though such a thing probably does not exist!!!

Thanks for the help!