I have a 2003 Mazda Protege 5 with over 130K miles. Yesterday, I noticed a rumbling rattle coming from under the hood (driver’s side, I think) while idling at a stoplight. The noise increased to a loud engine roar as I accelerated from the stop light, but then quieted down after reaching about 2K rpms. Since yesterday the noise has only gotten worse. It is now paired with a rythmic thumping that sounds a bit like a horse galloping. The car refuses to make the noise when I park it, let it idle, and open the hood. All the belts are tight and fluid levels are fine. For a little car, I sound like a semi-truck when I accelerate from a stoplight! The rattling noise is not metallic or like a squeal and it does not change frequency as I accelerate. It sounds like regular BUT LOUD engine noise with a rythmic thumping behind it. I would think it was the muffler if it sounded like it was coming from farther back in the car. I’ve seen threads pointing to the VTCS system and wonder if that could be the culprit, but I’m not sure.