I have a 2002 Hyundai Elantra with just under 90,000 miles. It recently started making a high pitched noise when idle after the car had warmed up. I’ve also noticed what sounds like water or some other liquid slushing around when I accelerate away from a stop, and sometimes on first starting up. The belts were just replaced & did not fix the problem.
I believe that you have two separate problems.
The sloshing water noise is likely to be an accumulation of water in the air conditioner’s drip pan inside the dashboard. The water is supposed to drain, but when the drain hose is clogged, the water accumulates behind the dashboard and can actually overflow onto the carpet, your feet, etc.
The accumulated water can also lead to mildew conditions in the car, so I would suggest having this attended to a.s.a.p. Your mechanic can clean out the drain hose with a blast of compressed air. Or, if you are so inclined, you can put the car up on jack stands or ramps, crawl under there with a length of “Weed Wacker” nylon filament, and poke the filament into the drain hose to clear it.
As to the high-pitched noise, I suspect that you have either a vacuum leak or a loose manifold (either intake or exhaust). If you are not mechanically-inclined, this will require a visit to your mechanic.
Check your coolant level. I think the sloshing you hear is air bubbles in the heater core from the coolant being low. Not sure what the whine at idle is. Does it change or go away when you step on the gas?
The noise only comes after the car is completely warned up (after 20+ min), and comes then when idle or under 15 mph
Would there be liquid in there if it hasn’t been used in months?
If you have ever run your windshield defroster/defogger during the winter, then you have been using the A/C–even if you were not aware of it. On most cars, the A/C compressor runs when the HVAC system is operated in defrost/defog mode.
The sound goes away when I accelerate
You really need to provide some clarification for the rest of us.
In your original post, you refer to two distinctly different noises.
Then, in subsequent posts, you refer to “the noise”.
When you tell us that, “the noise only comes after the car is completely warned up (after 20+ min), and comes then when idle or under 15 mph”, and that, “the sound goes away when I accelerate”–are you describing the high-pitched noise or the sound of liquid sloshing?
I have three degrees, in various disciplines, but none of them is in mind-reading.
;-))
Check for a vacuum leak under the hood, or possibly even the seal on the compartment where your air filter lives. I think the high pitched noise may be air whistling through a small crack or orifice.
Post back when you’ve checked the coolant. I’m fairly confident that you will find it low, but just having air in the system can sometimes cause this too. The next question if you are low, is where did the coolant go?
The coolant was low, we found there to be a leak from around the seal on the hose end running out of the engine area. We cleaned it, added some sort of sealer (I’m not exactly sure what it was, but the person who did the work does) and then re-clamped it with new clamps.
As for the HIGH-PITCHED sound - more closely related to a whistle, it is coming from the area around the belts (which have been replaced)…more towards the front, passenger side of the car. It stays at the same pitch, doesn’t go higher. As soon as I accelerate, it goes away. Everything else with the car is running fine.
What? You didn’t take the Mind-Reading class?!?! 
(sorry, I knew which noise I was referring to - forgot to think about everyone else)
If you are sure that the noise is coming from the area of the belts, then the next thing to check is the bearings on the tensioner and the idler pulley. After 90k miles it is very possible that those bearings have gone dry. That is exactly what happened to my car last year at around…90k miles.
I didn’t take a mind reading class, but I did take gibberish as a second language
I still think you may have an air or vacuum leak. As VDCdriver said, it may also be a belt, but this will usually change in pitch as the engine speed varies. You can have your mechanic take off the belts temporarily to see if the noise goes away.
thank you for all your help. i will be taking it to a mechanic AGAIN about the noise, now i have some suggestions for things to be looked at (the last place i took it just tightened the belts the 1st time & replaced them the 2nd). i’ve had a friend of mine who is good with cars take a look at it. i told him about the vacuum leak, he didn’t think the sound was associated with that, he had mentioned something more along the lines of a bearing (and I hadn’t mentioned that to him), but i will talk to my new mechanic about it. will post back once it gets figured out. thanks again.
I’ve taken the car to another mechanic who is having problems locating the source of the High-Pitched Noise. They took off the belts & the noise still comes back, and they’ve examined the parts around that. They said it is possible it is coming from the top part of the engine, but are going to keep looking around before resorting to that.
The sound of liquid sloshing around still comes back infrequently (however, the mechanic hasn’t been able to hear it) but they checked the drain hose and it is clean. I hear that sound, while I’m in the car, coming from the passenger’s side.
Oh the frustration!!!
The “sloshing” may continue for a while until all the air is out of the cooling system. There may be a small air pocket trapped in the heater core from the coolant being low. The heater core is on the passenger side, so this makes sense.
Have you managed to find out what is causing the high pitched noise when idle after the car has warmed up. I am having the same problem with my 1994 Crysler LeBaron that has 21,000 miles on it. My mechanic has not been able to find the problem and I really don’t want him to start tearing my engine apart to try and find it.