My '98 Honda Civic EX (manual 5-speed) is currently at 200,000 miles and makes a gargling/metallic rattling noise when I accelerate from low RPMs or whenever I’m climbing a hill. The noise sounds as if loose change is rattling around somewhere in the engine. I started noticing this a few years ago, when it was very slight and it only happened in second gear as I climbed a steep hill. Over time, it started happening in third and fourth gear, and now it is in all gears when climbing a hill or when accelerating quickly on level ground. I also notice a lack of power when this noise is occurring. In the past, if I let off the accelerator just a little bit, or if I greatly increased on the gas, the noise would disappear. Now, increasing on the gas makes the sound worse.
I asked my cat for advice, but she wasn’t very helpful.