Did you check the oil?

Engine knock under stress can also mean the the connecting rod is worn from lack of maintenance (oil change/lack thereof) and will soon fail.

If it's new, take it back. This is not typical and probably indicates a bad engine. If used, check engine mounting bolts. One possibility is the compression release is not working right and staying open. This will produce a loud knock and result in reduced power. When I start my mower with Tecumseh engine you'd swear it was about to throw a rod, but it's just the compression release staying open until about 1500 RPM is reached, about 1-3 seconds. On yours, maybe it doesn't go all the way back to normal operation. Whatever, it's not right.

@baymee:

Oil is fresh, drained it to change the blade and refilled it to the top with no change in behavior.

@saxman1:

It's used, I didn't have a chance to check it out on grass only ran it on pavement, didn't realize that a mower would sound so different while mowing vs idling on hardpack. The motor seemed tight when I was tearing the top end apart but I'll run through and make sure the mounting bolts are secured still.

Where would I find the compression release valve, and how would I go about testing to see if it's operating properly?

You can get a knocking sound when the mower gets choked up with grass under the deck. Is the under deck clean of dried grass? Are you cutting or mulching a lot of tall grass?

There is no 'compression release valve' - it's an internal system that keeps the exhaust valve cracked until a certain RPM is reached, for easier pulling when starting. But chances are it's not this - I just listed it as a remote possibility. It actually seems to me that it's knocking under a load which would indicate the rod bearing. This sounds like a rattle at 3200 RPM, not really a 'knock'. Have you tried the spark plug to see if it's snug, and the head bolts?

@saxman:

I'll check out the spark plug and the head bolts to make sure they're tight, hoping it's not a rod bearing but it can't be too much more difficult that doing them on a large motor.

@roadbike:

Honestly I'm not sure whether it's in mulch mode, never had a mower that had the option so maybe this whole time I'm being an idiot and have it set up incorrectly... I'll check that out first.

Just my 2 cents SR although Sax has offered very valid options to verify , I would tend to agree with Roadbike that perhaps you have not ensured that the bottom surface of the mower deck is clean and while mulching the clippings are becoming choked up under the deck . Also you do have the mulching blade installed correctly ? Have seen in the past where new users have actually installed a mulching blade upside down lol . Just had to ask :) I would hate to think that oil starvation has caused bearing fatigue within such a newer mower :( .

I replaced the blade the same way it was installed when I got it so I guess it's possible that it's installed upside down if it was when I first picked it up, guess I should check to see what the rotation is and make sure that the blade side is cutting into the rotation instead of the mulching side.

A blade is always installed with the beveled side of the cutting edge facing up.

Turns out the guy I bought it from had made 2 mistakes causing the noise. First off he set the front right wheel low and the rear left wheel high for some reason, second he put in 30 oz's of oil instead of the 20 it takes to fill it so the engine was having to work twice as hard to turn over.

Drained the oil and re-filled it to the right level and fixed the height issue now it purrs like a kitten and cuts great.

Only problem I have now is the bagging system kinda sucks but I'll see if I can rig up something to seal the bottom of the bag a little better.

Thanks everyone for your help, appreciate the tips to get me through this.

Well, that's one for the X-files. Never heard of oil overfill causing a knock, but it's good to know as a possibility.

Wow...if you had enough oil in the case to cause a knocking sound it must have been real full. Like when a cylinder get full of gas.

Glad you got it sorted out and the mower runs.

Me too, can't imagine what the guy was thinking to put half again as much oil as was called for in there. You'd think when he looked at the dipstick and saw the oil level was 2" above the line that says FULL he might have gotten a clue that something was wrong.

Thanks again

I had a guy bring in his nice walk-behind mower for me to fix it. Said it was hard to pull the rope, and the only thing he did was change the oil. Said he filled it to the top! Turned out-he meant the top of the dip-stick tube!
Well, i drained it all out, but all of the inner passages were clogged with oil, and even after setting for several weeks--no start! I took it in trade for one that runs.