We have a 2005 Toyota Echo, manual 5 speed, with 160K miles. Everything works fine except for when you are accelerating (can happen in any gear) and mainly uphill. I assume the primary condition is when the car is being pushed or under more load than just cruising, anyway, it will will make a low grumbling noise and simultaneously shudders as if it is going to stall. This happens in a jerky fasion and doesn’t last very long but consistently under aforementioned conditions. We have 3 Echo’s by the way, all manual shift (Echo friendly 
Scratching my head with a wrench in Florida!
How are the basic maintenance items? You’re probably just looking at a basic ignition or fuel flow problem. Old plugs or wires, clogged fuel filter, failing fuel pump - stuff like that. Partially clogged exhaust is another possibility though not as likely as the others. So check out the plugs and wires and stuff and test the fuel pressure - but do it under load - and give it a new fuel filter.
Thanks, I changed the plugs 2 years ago, kept up with oil changes since we bought it in 2005. Haven’t changed anything else except front wheel bearings, brake rotors and pads. I recently switched from conventional oil to synthetic blend. I also have been adding some Marvel Mystery Fluid to the gas and crankcase occasionally. I am leaning towards fuel filter/pump. Thanks again.
Should have asked how old the clutch is. If the clutch is original you might just be getting some slippage with old, weak, rattly dampener springs.
Hmmm…it is the original clutch. Would explain the sound, drop in acceleration and it only occuring on hills or when being pushed to perform. Thanks again, I was starting to wonder if it was the marvel MYSTERY Fluid. I think you are narrowing it down!