Rumble at certain acceleration

Hey all,
Back again with some fun. By the way, for those who did help me out the last time, I did get the stereo installed.
Anyways, something new popped up last night and I wanted to ask about it. I
ahve a 2001 Focus with automatic transmission. Last night, I drove to a restaurant and had no issues, but on the way back, I noticed a rumble while accelerating. This was something new. It didn't start up immediately, only after driving a while. Anyways, I can accelerate past the "spot" and it stops rumbling or slow up and it stops rumbling.
So I let it sit all night and tried to do it again this afternoon. One the way to my destination, I couldn't make it do it, but I left the car running and on the way back, it did it again. My thinking (and I am NOT a mechanic in any way) is that it could be low transmission fluid... I haven't had it in for an oil change in a while (big mistake), so the transmission fluid probably hasn't been checked in a long while. Anyways, does this sound plausible? I'm thinking that I'll go to jiffylube or whatever tomorrow and get it checked. As I've recently been unemployed, I'm hoping it's not a huge transmission problem or I could be hosed.
Thoughts?
CK
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My first thought is: How many miles on the car? The transmission is part of the powertrain, and should have warranty up to 100000 KMs. When you get the oil checked, look at the colour and smell. If the oil is black and smells burnt, the tansmission is shot.
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The car has 42650 miles on it, and I'm checking on the warranty. In any case, I just got the oil changed today and the transmission fluid replaced at Jiffy Lube, so I'll have to check on the oil in a few days.
Well, just called Ford and apparently the warranty was to 36000 miles. That's a pain (I bought it slighly used, so I didnt have the option to go with extended)
CK
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If you are able to open the hood you can check it yourself...it's in your owners book as to what to look for...also when you wipe off the dipstick look at the color and smell the fluid in it - even if you can't change it yourself you can still look...even I can do it. Linda
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