funny noise from engine

i have an aftermarket exhaust on my 01 focus se SPI and when im crusin bout 50 60ish and i keep the gas juss enough to keep the car goin the same speed. through the exhaust i can hear a weird tone from the engine, its like its goin in and out. kinda like a mellow chug. or deisel effect....any idea's? it runs like a champ...

i have an aftermarket exhaust on my 01 focus se SPI and when im crusin bout 50 60ish and i keep the gas juss enough to keep the car goin the same speed. through the exhaust i can hear a weird tone from the engine, its like its goin in and out. kinda like a mellow chug. or deisel effect....any idea's? it runs like a champ...
check the egr tube going into the exhaust manifold on the back of the engine.they have been known to crack where it meets the manifold and cause an exhaust leak.