This started about one month ago. I heard very loud noise after I started the car. Then 2 min later, it became normal. I had thought it was because of the weather was too cold those days(0F) and it was the sound of fan.
However, I keep hearing loud noises in very warm days(55F) each time I started the car. And the noise sounds abnornal for me and I found it comes from around the engine. This noise is different from before. the noise now is uneven. The strange thing is after driving a while, everything becomes normal.
There are seval possible reasons I can think of now
1) I added the different type of engine oil to engine. The two oils reacted with each other which lead to the thickness of the oil.
2) the level of oil is a little bit higher than high level.

Please Help dignose. Is this noise mean the engine have big problems?
thank you so much!

I have had the strange noise in my engine in the past. It appeared to be a bad timing component that eventually broke my timing belt. So do not leave it without attention. You probably can use mechanics stethoscope. Or take it down to a professional.

High oil level is bad for your engine: it calls foaming which actually reduces oil level. So your berarings may not get enough lubrication.

Sam

Thanks a lot for replying. I did have the timing belt changed 10,000miles ago, which was two years ago. I don't drive that much.
recently, I have the battery changed. The power steering oil is leaking somewhere. I don't feel these can cause the loud noise.
If it's because of timing component, is it because of the loose or something else?

Very tough to say without more information. Even then it's iffy. Your best bet and quickest path to a good fix is to have some knowledgable ears listen to it. Noises are about the toughest problem to troubleshoot from miles away.

More info? The only discription of the noise is "a fan" and "uneven". The location you give is "the engine".

Which side of the engine?
At idle, driving down the road, or both?
Does noise speed up/slow down with speed or engine RPM?
Just in turns, driving straight, or both?
Is it a cyclic click like a stick in bicycle spokes speeding up and slowing down?
Is it a steady moan?
Everytime you drive it?

Mixing different viscocity oils isn't a good idea. If you're mixing 10W30 and 10W40 or similar, it's not a major problem. If you're mixing 10W30 and 20W50 or similar, I would change the oil and stick with one type.
Too much oil can be just as bad as not enough. Too much oil can create air bubbles in the oil which dereases lubrication.

Thank you so much. I will try to answer the questions you listed above. Hope you to figure out the problems. By the way, My car: 93 camry XLE

Which side of the engine?
Sounds like the back side of the engine close to window.

At idle, driving down the road, or both?
both, but disappear after driving 10min.

Does noise speed up/slow down with speed or engine RPM?
No.But louder

Just in turns, driving straight, or both?
Both

Is it a cyclic click like a stick in bicycle spokes speeding up and slowing down?
no

Is it a steady moan?
steay but higher pitch than moaning. it's like (oh...... for 30sec, then stop, then start again) I am so sorry I can not describe the sound that well.

Everytime you drive it?
I only drove the car twice and I heard this twice, First time, only first few minute under parking I started the engine and it disappeared when driving. Then yesterday, It seemed to have this noise when accelerating during the few minuters driving down the road.