Engine Mechanical problem
1995 Cadillac Seville V8 Front Wheel Drive Automatic 72,000 miles
Starts, warms up and idles fine. Good gas mileage and performance. No hesitation.
After listening to different components with a wooden dowel, the only noise comes from the forward valve train but only under light loads in the lower RPM range. I get it while cruising constant speeds, with or without cruise control. The sound is reproduce-able while applying a gentle power brake. No codes. EGR valve and solenoid seem to function fine. Removed valve cover to inspect. Nothing out of place or loose.
Being a new owner, I was caught by surprise with a low oil level at about 50% oil life. It had 10-30 synthetic. I added a quart+ and that's when the noise began. Very slight at first then after about 800 miles, it's kinda loud. Changed oil to a higher viscosity 15-40. No change.
Can be kind of difficult to diagnose accurately, it sounds like you have oil itself ruled out. There are several possibilities for top end type noise. Low oil pressure, specifically to the top end could be a possibility. The harmonic balancer clamping drives the oil pump, so if the balancer bolt is not tight enough it can allow for some slippage, that would be something to check. I have seen some lower end repairs that have created issues for top end oil pressure, one of which was where someone had sealed the oil pan without removing the transmission/crossover, and in the process had distorted the oil pump pickup tube, allowing it to draft air and result in top end noise. There are oil gallery plugs in the cylinder heads, but the are difficult to access. The front bank is under the water pump housing, and the rear bank is obstructed by brackets and hoses. There is no spec for pressure at these pressure ports, nor any procedure for checking, it was kind of an afterthought. It could be a faulty valve lifter, or follower(which I believe this era had bucket lifters that rode on the cam lobe itself). With it being a used car, you have little or no prior repair history. I would suggest removing the belts(main and water pump), and seeing if the noise still exists. Be certain that it is not an exhaust leak at the manifold. Checking the balancer torque, not sure on the exact spec, but with the base torque plus the angle, it is over 200 ft lbs. And then consider how far you want to get into it. Before I went to far, I would have a second opinion perhaps. I am not questioning your mechanical aptitude, but I don't know it either. Having not being able to hear the noise myself, it is a just a suggestion. If you are certain it is a valvetrain speed type noise, then go with it. If there is a chance it could be carbon in the combustion chambers, approach that first as it is possible as well, and somewhat simple to rule out.