98 Corolla: Very low RPM noise

I've noticed that at very low RPMs (but not at idle), when I apply very little gas the car makes a louder grumbling noise. This grumbling sound goes away when I apply more gas. This sound is never heard at any other moment at higher speeds.

Is it my automatic transmission kicking into first gear? I have about 135km on the car with no transmission change yet.

Will it break soon? expensive repair?

i get this exact problem on my 2002 corolla (130K). and my sister's 2001 corolla (60K) just started doing it too. what gives?

*bump*

any ideas or things to check myself would be much appreciated. thank you in advance!

I had the same problem on my 99 corolla... kept driving it, and then heard a clunk, and it wouldn't start. Towed it back with my truck. (thank god for the twenty dollar tow ropes at harbor freight.) I just popped off the valve cover today and it looks like my timing chain is loose as hell... maybe the clunk was it jumping a tooth.... looks like I'll be breaking it down so more in the coming days. (140k miles)