Clunking sound

It happens only with acceleration, and only between 20 and 30 mph. If I take my foot off the gas, the noise will stop.
It does not happen while braking.

The mechanic (who is at wit's end) is thinking MAYBE it is motor or transmission mounts. He suggested it could be the drive shaft, but the sound would be coming more in the center, and back further. Presently the sound is under the driver's feet.

My friend, whose car this is, says the mechanic has checked all these things. He has a good reputation and refurbishes Subarus for a living. So it's a little odd to have him stumped. (She just bought the car from him and he promises to make good on it.) I just thought we might get some new ideas by putting the question up on a forum. I understand it's a difficult thing to diagnose from a distance. Thanks for taking the time to respond.

OK. I'm the one paying, and I just may have the car taken to another mechanic to look it over. The problem is that the sound doesn't happen every single time. And the current mechanic is telling us we are going to have to wait until it happens all the time. My worry is that if it is serious and we wait, something that is about to break, breaks at the wrong time. He says not to worry!

What? Not only does that sound dangerous, who get's to be looking under the hood when the car lurches forward? I don't get it. Of course it will jump forward, or am I missing something? Or you mean to just see if the clunk happens? Why the hood open? I'll try it if you would explain it a little more clearly. I'm slow!
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I do have some new information though. I walked into another mechanic's shop, to see whether I could get someone else to look at it. He's the guy I go to and something of a genius. I barely got the story out when he said he's seen this all the time with Subarus. That there are bad replacement axles being made these days. And he's had to replace replacements multiple times. He said the problem is in the CV joint, and the joint being sealed he couldn't inspect it anyway. But he seemed very confident it is that.

So now we're kind of stuck because the mechanic who sold it to us, will honor the warranty but doesn't want to work on it until the sound is chronic. We haven't presented him with this new information yet though.

OH! I'm sorry, I thought you meant see if the CAR jumps forward! You mean the engine. Interesting idea. I don't believe that was looked into. We'll try it. I'll let you know.

Sorry, I haven't posted anything because nothing has developed. I described to my friend the test that Ken had described, and she refused to do it (I guess thinking she'd feel bad if she ran me over by accident). And she's been busy at school - which she commutes 40 minutes away to, and interestingly hasn't complained about the noise in the past couple of weeks. She basically is willing to follow the demand the mechanic who sold it to her, and wait until it happens more often. I'm leery of that but she says the mechanic doesn't think it is dangerous. I will post something whenever this gets resolved, but I have no idea when that will be at this point.