Weird Knock from Contour SVT?

I have a 98 Ford Contour SVT that has an odd knock. It knocks at different rpms and sometimes it don't knock at all. When you rev the car up very quickly you don't hear it knock, but if you keep the rpms steady it starts to make a knock, and it happens at different rpms, sometimes 2k or 3k. This sound is very nonlinear with the car. It sounds like horses galloping under the intake manifold. Anyone have any ideas on what it could be?

Loss of power in a 1999 Chevy Blazer 4.3?

I just bought a 1999 Blazer 4.3 and for some reason it has a hard time picking up speed from a dead stop! The transmission is not slipping and the cat converter is not clogged up! It does have high miles, 177,000 to be exact! But I had ford trucks with more miles that more power! Is the engine just tired or it it just geared slow? The tires on it are 235-75-15 but I can't see that would make any difference, would it? The engine sitting and running sounds like a brand new one and it has all kinds of power once you get started out going down the road! The only thing I can come up with is just too many hard miles on it! What do you guys think?

Ford crown Victoria police car?

I have a 2011 Crown Victoria. I bought it from a guy who uses it as a show car. The car is fully loaded, sirens, lights, police decals, Bullard in front too. I'm a little anxious because I love the car but I don't want to go through the trouble of removing everything plus I like the look. I never use the equipment to even use it you need a key for an inline key switch I know it would be impersonating an officer if I pulled someone over but can I get in an any type of trouble just for having the equipment as display? I'd really hate to have to remove it

How often should I change my stock air filter?

I own 2011-2012 Ford Crown Victoria and I live in Riyadh, Saudi Arabia, which is known for the huge amount of dust in the air, to put it into perspective a car would gather the same amount of dust on its surface in one week the same as a car in any "normal weather" country would have gathered in a month or two.
So should I change in the manufacturer s recommended service time period or follow some short time period, even though it does say in the manual if operating in dusty conditions check frequently and change if necessary but I'm not sure how necessary is necessary. One more question; if operating in these dusty conditions, is just blowing pressurized air onto the filter (when it's out of the air box of course) enough?

1996 Ford Contour revving up and down problem?

96 Contour 2.0.
-Engine rev's up and down at idle or at a stop, with or without a/c on.

Mechanic thought egr valve but i clean it and same problem. What else can it be?
car has new timing belt and maf sensor.