odd idling noises/behvaior

Hey all, my 2001 is manual. Recently ive got odd noises/behavior. It idles at standstill sounding like there's very minor pressure on the gas pedal. The engine sounds like its working a little bit, more than I remember. If i put it in neutral and brake, in the 15-30mph range (still neutral and braking) the engine starts into this loop of: revving harder, then softer, then harder, then softer. For someone who knows nothing about cars/mechanical work, the closest thing i can relate it to, is how a crappy old lawnmower sounds when the engine struggles to start or stay on. It revs harder then softer and keeps alternating every second or two. But while neutral and braking, once I get below about 15mph, it just stays on the high end of that loop, sounding like the engine is working harder than it should. Any ideas or info would be greatly appreciated, thanks.