4.2l noise

[E150]

Good morning. I have a 1998 E 250 4.2l V6 , 4EOD tranny. I have not read every post on this forum, but searching for 4.2l engine noise sure gets you alot of reading material ! Here's my noise problem for the experts. This motor has 15,000 on a rebuild. A crankshaft kit with connecting rods, new pistons, rebuilt cylinder heads, and upper and lower intakes decarboned. My noise started around 8,000 miles. The noise does not sound like rod bearing noise, but the frequency does. It's more like a loud pinging or possibly knocking. The noise happens only under acceleration and with the slightest release of the throttle the noise instantly goes away. It does not happen at all revving the engine in park or neutral. I just found a very interesting thing this weekend. I tried to power brake and run the revs up in drive and could not get the noise to occur. However, when I power broke in reverse the noise was very much there. Very predictably there. I'm now not sure if I have an engine noise, tranny noise or a strange rattle, (exhaust?) Any thoughts anyone? Thank you in advance.

[bokker]

hi, i have no explanation but it is worth checking that engine lift brackets are not loose as i have read that some time they rattle. bokker.

[BrianA]

If you put the transmission in a different gear when the noise is occurring does the frequency change? If the noise occurs at 3000 rpm in drive, does it occur at 3000 rpm in second and low?
Trying to get to this:
Is it related to engine rpm, or transmission rpm?

Can you put the van on a lift and recreate the problem?
If so, a mechanic's stethascope might just be the ticket to isolating the source of the noise.

(on a side note- a very under utilized tool in my opinion.)

I suspect it might be torque converter / transmission.
How many miles on it?
Have you kept the fluid changed in the transmission?

edit to add: some reports of engine lift bracket bolts working loose and rattling.