Most likely a rattle from that area in the engine bay when cold that subsides somewhat when engine gets warm.
The altima uses a timing chain, not a belt.
Rough idle, loss of power, poor mileage.
Hard starting, loss of power, excess fuel consumption, check engine light, and a blowing sound under heavy acceleration.
=
back fire ,disolin
Your timing MIGHT be off if: low power, poor acceleration, poor gas mileage.
There are no signs or warnings. It can look great and break the next day. That is why they are changed at a specific interval.
There is a song from the late sixties or early 70s that has a line/chorus: "Signs signs, everywhere signs, blocking out the scenery, breaking my mind; do this come to that can you read the signs" I dont kno who or what the name is , but someone else should
The car engine will be totally inoperative!
When you crank your motor and it makes a high pitched wineing sound
If it runs at all,the signs will be fuel starvation when loading the engine : climbing hills, trying to pass, etc.
There are a few numbers that do not need signs. You do not have to sign when timing something.