99 silverado exhaust vibration/grone help

i have a 99 silverado ext cab with the 4.3 v6. the truck makes a bad vibration only at 20 & 40 mph. at first i thought it was the trans and i had a friend rebuild it after i bought the truck. still did the same thing so i took it to several dealers and a guy found a service bulliton stating to fix the prob they had to put a flex pipe on the exhaust. worked for about a week not bad since the part was $300.00. i have since taken it other places and it is for shure the exhaust making the noise. i looked under the truck and the exhaust bracket is bolted to the trans brace making you feel the vibration through the trans. i cut the bracket off the trans and made my own bracket and mounted it to the frame. made the vibration go away but now makes a grone sound and is very load. anyone have this prob and know of any fixes? thinking of puting new exhaust but don't think it is going to help. thanks for the help:banghead:

its a vibration the engine produces, the only way youre probably going to get rid of it is to sling the exhaust with rubber.

sling??? you mean rap it??

I'm confused?? Do you have a stock muffler?

I still have a mysterous hum coming from underneath too. I took mine to a dealership where a friend works, and after 3 days they could not find the sound. Of course, it does it really bad when cold outside. During the time that I took it to them, it was much warmer than normal. So, I got it back and looked at it myself. I can definitely hear it through the frame at 20-25 miles an hour. I need to put it back up on jack stands and get a section of hose to listen around. I personally think it is the center support bearing, but the dealership did not think so.

the truck has stock single exhaust. the only modifcation is from me cutting the bracket off the trans mount and turning it to the frame cross member which changed the vibration feeling to a load groan sound now. and the dealer did put that flex pipe on but that didn't change anything. what do you mean put the exhaust on rubber straps. should i remove the stock brackets and put on new ones? thank you guys for the advise.