WHen my 72 velle is idoling it is making a wierd noise. It will idol smooth for about 5 to 10 seconds and the then I will get this low rumbling noise. Almost like a vibration. While it is making the noise the Oil Pressure drops about 3-5 PSI and my voltage drops. It is almost like the engine is slowing down for some reason. Can anyone offer any ideas what I can check? Thanks for the info.