1990 Cadillac Eldorado
 

I have a 1990 Cadillac Eldorado with about 71 500 original miles I bought it 2 weeks ago it had been sitting for 3 years was last serviced at a Cadillac dealer in Las Vegas at 70 893 miles then parked. After fuel pump new fluids she ran perfect no codes passed emissions I have been driving her and she is a head turner. There are no codes in the system everything in and on the car works.
Here is the problem the car has a noise in the motor I think lower end not sure exactly where but it sounds like a knock. performance is perfect the noise does come and go it is weird. mostly there when I drive on the freeway and it is really warm then you can hear it under a load at lower RPM it goes away when it is in neutral, can anyone offer help in diagnostics and if so if it is main bearings if it can be done with motor in the car.