I Hear a strange whistleing sound

Question: I have a 99 Hundai Elantra with 124,000 miles, just change the oil to high mileage oil, car started to run odd, checked under the oil cap and found yellow gunk, nothing in the dipstick though? there is also a strange whistling sound the happens after 20 miles per hour mostly when stepping on the accelerator, do you know what could be cause of these two problems? Answer: The yellow gunk is nothing to worry about, probably just some condensation. Clean it of and replace you pcv valve and it shouldn't come back. As for the whistle and running different. This is an indication of a vacuum leak. Find and fix the vacuum leak and it should be fine. Since you just had the oil changed, check things that might have be left loose of disconnected. If they checked your air filter, check the connection, hoses, etc. You can run engine in park, pop the hood and rev the engine until you start to here the whistle and try to locate the vacuum leak that way.