Transmission Thumping??

[athenshome]

I have a 99 Navigator and have been recently noticing a fairly heavy thumping that feels like it's coming from the transmission. It only does it when going uphill in 4th gear (I believe) at lower RPMs (noticed it this morning around 1500-2500 rpms). It is a heavy enough thumping/bumbing feeling that passengers would notice and it's get your attention. The only way to pass it when it does this is to accelerate heavy and push through it.

Something is not right about this thumping and I need some help figuring out what's what. I do not have any check/service engine lights on and have not had any issues mechanically. The Navigator has 188k on it and the transmission has been serviced in the last 20k miles.

Does anyone have any ideas as to what this may be and where to start looking? Many thanks in advance!!

 

[LithiumCobalt]

My first suspicion is the transmission torque converter. Was genuine Mercon V fluid used for the flush or was it a generic fluid? Mercon V is the only stuff that will have correct friction modifiers in it for the torque converter to operate properly.

My next suspicion is that you might a slight misfire condition. Not enough to set the CEL, but enough to get the computer systems hot and bothered causing the possible torque converter chatter. Could be dirty, worn plugs, worn coils or a combination of the two.

 

[athenshome]

I believe it was filled with Mercon V fluid and not a generic. Generic fluids is unlike the mechanic who did it last. I did scan the computer this morning to see if any codes would kick up about a misfiring cylinder or anything and nothing showed up. What's the easiest method for checking misfire?