C indicates the oil spec for diesel engines. Diesel engines require lots of detergent and cleaner additives to deal with soot. These oils are also rated for gas engines and will clean them out very well.

My favorite example is Chevron Delo 400, but equally good for your purpose is Mobil Delvac which can be purchased at Wal Mart.

My guess is that you need to change oil more often.

Keep in mind that it is somewhere between very difficult and impossible, to diagnose noises over the internet, but cleaning out the engine with such oil would be a good first step in fixing the problem.

Please let us know the outcome.

Good luck,