My car has a hydraulic knock what is it

See if you can identify where under the hood the noise is coming from. If it's truly an engine noise and not a device attached to the belt, depends on whether it's a piston problem, or a stuck, collapsed or plugged hydraulic lifter (both can make a similar noise). Varnished or dry hydraulic lifters are usually the problem, but only a qualified mechanic can determine which it is, and if it's a piston, it can cause serious engine damage.

A Defective hydraulic valve lifter will make noise, it would be tapping noise, they can be very loud, but I would really need to hear what this is to determine the cause.

Most probably the lifter has gone bad, but if it is a LOUD knocking sound, it is possible one of the crankshaft / connecting rod bearings has been damaged. Usually a bad valve gives more of a "clicking" sound.
A bad camshaft lobe will also cause a "knocking" sound usually with an engine with high mileage.

5.000rpm's high revs and you run into problems with valve bounce ( hitting pistons) ,ring slap, gudgeon pin knock, big end noise and possible main bearing noise . You may also be getting a hydraulic lock in the hydraulic followers that is holding the valves open too long. Unless the engine has been specifically built for these rpm's any loose ,worn on unsuitable part will cause problems I am thinking that you have around 100.000 miles on the clock after 7 years and the motor is not young Ford V8 engines specifically built for a 500 klm race are limited to 5.500 rpms because they destroy the timing chain if driven over that speed .

Have you checked the oil pressure.

Remove the engine drive belt and run to eliminate any accessories causing noise.

Check valve rockers, could have a bad hydraulic cam follower that is not pumping up.

Could be more serious, why did the head gasket fail, might be an associated problem with that if engine overheated ect.

No if the oil light has flickered you have oil pressure loss some time just belive the sensor do help us out. lifter do not cause oil pressure loss pressure is built in the rotation of the crankshaft. The knock under the plenum will point you to that cylinder that is affected, (main bearing) as for a 1995 the injectors can only cause a misfire but no knocking noise,will they honor the warranty

The knock sensor picks up vibrations from detonation and sends a signal to the engine computer, which then adjusts the igntion timing [ retards advance ] to correct it. You can read the operation of the sensor with a full function scanner, if it's working then you have to look for a physical problem with the motor causing the konck.

if this engine has hydraulic valve lifters, then it could be a weak hydraulic lifter that pumps up once the oil warms up. Hopefully you dont have low oil pressure. Does the oil light stay on for a while after you start the engine. If it does that could cause your engine to knock

probably the oil pump

maybe a hydraulic lifter or a central bearing.... do you hav trouble accelarating at all? if its they hydraulic lifter, you will be fine for about 90k miles and even then nothing serious will happen, its just annoying. if its the central bearing on your motor, then you will have anywhere from 0-3000 miles left until your engine shuts down on you and your car stops moving. for me it's the hydraulic lifter witch triggered the knock sensor, which is there to detect knocks for oil related problems but unfortunaetly picked up my knocking too and thus went into its "energy saving" mode and slowed accelaration down ALOT in order to protect my motor. of course since it was a hydraulic lifter, this was pointless to me. if this is your case just take it to a mechanic and pay $40 to get the knock sensor unplugged. this won't solve the knocking itself but it will allow the car to accelarate at w/e speed you want. THIS IS NOT the solution to every knocking/ticking problem. there are MANY different ones out there so the best way is to get it checked out by a mechanic.