Eng Noise, help

Provided oil is nice and clean, guess would be the oil pump is dropping pressure because its going bad or it's not being spun. Personally, I'd break open the motor, I just prefer to see what's going on. If you want to poke around it a bit more first though, you can take the belts off to get less noise and listen in. My personal guess is when the oil drops the lifters are getting a little sticky. Sorry I couldn't be much more help.