1997 Kia Sportage engine noise

Hi my 2000 kia makes an annoying noise coming from the engine. Sometimes the noise appears from the start in the morning, other times, the car runs fine and suddenly when a traffic light gets you and you try to go again is when comes it on. At higher speeds is not noticeable and more common during hot weather though lately here in NJ has been cool and the noise is still there. I was told it was the water pump, fan clutch and a/c compressor by different mechanics, but after changing all these parts the noise is still there. If the car is iddling like when park is not making the noise, but if you rev the engine while park it will come. When the noise is not there the car runs fine and feels light and easy to accelerate, but when the noise comes the car sounds as if it is struggling. The transmission shifts fine even when the noise is on, the car has no leaks and burns no oil. The noise sounds similar when a big tarctor trailer is going at low speeds, First time I heard it, I thought it was the noise coming from a truck next to me on the highway, but soon I realized it was coming from my car. The check engine is never on during this episodes and the car sound s a s if it is struggling to go ahead, sounds as if something is stuck, sounds rough as if you were climbing a hill with a big load, it doesn't sound right and it is worse when the car is warm. In summertime, it does the noise right from the beginning after the car has been parked under the sun for a while, now is doing it in winter time more often too. Appears to come either from the center front part of the engine or from the front part of the engine on the drivers side mostly. Nobody can help me so far and I have spent thousands trying to solve this problem. The car is just not enjoyable with this noise going on and is not running light as it should. Help please.