Shortly after changing the oil (less than 60 seconds after restarting the engine) I noticed that my engine has developed a new noise. It's much louder than the valve clicking sounds that I'm used to. At any engine speed below about 1500 rpm I can hear it. It's most noticeable at idle. I've never heard rod knock first-hand before, but that's my best guess. Those few seconds without oil pressure after the change may have finished off one of the rod bearings. It sounds like a knock: not quiet, not really loud, but very noticeable, even with the hood down. The noise doesn't really change when disabling individual cylinders, just slows down with the engine. I used a breaker bar and listened at different locations around and on the engine. I wasn't able to pinpoint anything--it just sounds like it's coming from the block, or maybe the head (I can't think of anything in the head that could ever sound like this though). Can anyone think of any other possible causes for the noise to develop immediately after an oil change? The filter is the same brand as I was using previously and the oil is the same weight. Any advice is appreciated.