Elantra engine problems?
 

Hi guys, I have a 2003 Hyundai Elanta GT that has about 102k miles on it.

Over the weekend, I drove to work in the morning and when I turned the car off I heard a weird 'settling' noise, but it sounded like metal clunking really hard. The noise came again when I shut the door, but that was the last time I heard it, and I can't really replicate the noise good enough to explain it.
The next day {again at work} I noticed a funny smell when I shut the car off, something like gas. I noticed the tank had more gas so assumed that when my boyfriend had the car he had filled it up, but it continued to smell like gas after being shut off, and only on the exterior.
We drove the car around a little bit later in the day and both continued to smell the gas-like smell, but haven't been able to spot any leaks yet, and the gas cap is sealed tightly and it is closed.
Anyway today my boyfriend smelled more of a burning wire/oil smell and said it only occurred when the car was warmed and the heat was on. He opened the hood and found a sticky yellowish liquid on the engine, but when he checked all the fluids he said they were full.

We have an appointment on Friday {the earliest we can make, especially with our work schedules} regarding the car shaking at idle, and we're going to have the mechanics check out the smell too, but in the meantime, I was wondering what we could do to ensure the car doesn't break down, or if it is even safe to drive.
This is our first car and we bought it used, and we're learning how to fix car problems as we go, so excuse my lack of knowledge, and I appreciate any answers.

STOP DRIVING IT LIKE LAST WEEK !!

1 - rough run... MIL flashing at you ?? Misfire will kill a cat in no time.

2 - Wait till cool engine where radiator cap can be safely removed,, start engine, make sure coolant full in radiator and not bubbling/blowing out when engine started..

3 - yellow stuff... oil leak, dirt stuck to oil ?? Oil level correct ?

4 - Overheat a 2.0, you in world of hurt if not caught, or driven while overheated,, it'll twist the head, out-of-round the cylinders, warp the deck,, time for new engine..

No rough run, and the check engine light hasn't come on yet. Some internet research has come up with a possible blown head gasket but I should mention that we're getting the average mpg.
The coolant is full and it doesn't bubble or blow out.
We are thinking the yellow stuff is oil leaking, and I forgot to mention earlier we found the fluid on the exhaust manifold. We think it is being burned from the car's engine heat; the oil level is full, and the car itself isn't overheating yet.
If it's leaking onto the exhaust manifold, could this mean the leak is from the valve cover gasket? We did see a small leak underneath the car near the passenger side, but it wasn't much. How can we check where the leak is coming from?

UPDATE: It was the valve cover gasket and the orings - the motor mounts are worn to the point it's making my car shake at idle and certain points of driving. It broke my radiator just before winter, and apparently caused the break in the valve cover gasket.
Got the valve cover gasket fixed and making an appointment next week to fix the motor mounts.

Thanks for letting us know, I will keep that in mind if I see yellow stuff or idle shaking in my 05 elantra!