Engine noise continues.

Well this is just a guess. Jag Owned by Ford. hmmm. Could it be that the Noise issue is back? Well it might seem that you have noise from your head unit. Seeing that when you connected a walkman it was gone. Try this. Get a Wire 16ga. Connect it to your Headunit and run it with your RCA to your Amp. Also make sure that your Amp is Grounded WELL!!!!! Not to a screw in metel that is painted. If there is paint where you are grounded remove the paint and re do your ground screw. Here is why. The best place to establish a common Signal Ground is at the back of the head unit. Since the head unit has to be at one end of the system, and the amp is at the other end, it is a great place to establish the one-and-only signal ground. This is commonly taken care of automatically through almost all modern head units, by internally making the same reference to ground for both left and right RCA shields. The main goal is to make sure that this ground gets to float all the way to the amp(s), and not get misdirected along the way. The same signal ground seen at the deck must carry down through the rest of the components to the amplifier. If the grounds are different between any two components, ground noise can creep into the system. Most aftermarket autosound component manufacturers help you keep a single signal ground by floating the component's signal ground and allowing all grounds to reference back to the head unit. Sometimes, though, the signal output grounds will be chassis grounded on an individual component. If that component does not use DC-to-DC converters in it's power supply to isolate that ground, their signal outputs will be grounded and noisy. It is usually easier to substitute a more expensive component (with no chassis ground on it's output) than spend an hour trying to trace the whine. If there are several suspect components in line between the head unit and the amp, just take a long RCA cable and run it straight down the middle of the car (over the carpet, seats, etc.) straight to the amp. If the noise goes away, then it's time to check each individual component. One method of finding out which component is responsible is to bypass one component at a time using double female RCA barrel connectors. The barrel connector will float the outer shield of the RCA, preventing a second reference ground and a ground loop. If the noise goes away when you bypass a particular component, then you have found the culprit.

Hope this helps