I have a 97 civic ex. Completely bone stock under the hood.
Just recently my car began to make a noise, that the best discription I can give is like a frog croaking, really fast. I have been trying to trouble shoot it and this is what I've got. Any ideas?

Only makes sound while in gear, neutral sounds fine
Inconsistent (maybe because of the grade of the road)
When I start the car no noise, when I drive in first right after starting (no noise)til I shift to 2nd it'll begin to "croak" lol.
After that in 1st gear once I hit 3k+ it'll start to do it.
The noise has a plateu, as in it doesn't really get any louder after 4k, and sometimes even gets quieter.
When the noise begins I've put my head
towards the passenger side and it seems louder on that side.
I also noticed my engine started to leak a decent amount of oil, haven't been able to pinpoint where.

I'm not a mechanic by any means, but have tried gain as much info as possible in hopes someone might have experienced it before or seen the problem and know the solution.
Any help/input is greatly appreciated, thanks!