I have a 86 Iroc with rear disc brakes, new rotors, calipers and pads. When i drive for a while the rear driver set up makes a vibrating humming sound when i start in first gear. This only happens when hot. I also had changed the rear carrier bearings and that wasnt issue. My local mechanic cant figure this out

 

What happens if you start out in 2nd? (not that you'd do that in normal driving, butt ... useful troubleshooting data point maybe)

Does that brake seem hotter than the others?

Not to be a jerk (much), butt, is this a surprise? Not that your "local mechanic" is incompetent or whatever, just, you have an ANTIQUE car, and one of the fastest ways I know to go bankrupt is, to PAY other people to indulge in your ANTIQUE CAR hobby FOR YOU. Like the C3 Vette guys that go through the half-shafts / rear calipers / trailing arms cycle every 2000 miles OR LESS. Effff abuncha "local mechanic" and do the logic thing and figure it out yourself.

 

Ill try starting in 2nd
what do you mean calipers cycle every 2k miles on c3?

 

I guess you've never been around 60s & 70s Vettes very much

Back in the day, there were essentially ZERO foreign cars on the road. The Vette was BY FAR the most "exotic" thing most people would ever come across, and like today, it cost about TWICE what a regular car of some sort did, so for the most part only relatively wealthy people owned them. The Vette was never meant to be a daily driver; it was strictly a Sunday afternoon show-off midlife-crisis coverup for penile-challenged people who never got their hands dirty. So if you drove one very much, SOMETHING was ALWAYS breaking about them. Those particular 3 parts - rear calipers, rear half-shafts, and trailing arms - INVARIABLY fail within a few thousand miles. Their calipers were zero-th generation American discs, 4-piston fixed-caliper stupidity, the very first product of that kind, and their poor engineering showed it, ESPECIALLY the rears, and ESPECIALLY if you screwed up and drove it in the rain. The seals were terrible, so they got water in them and the piston cylinder rusted, and they'd leak and/or seize up, and couldn't be "rebuilt" in the field because the cylinders would be so pitted. Half-shafts are like front CV axles except that they were STUPID CRUDE in the 60s; the whole idea didn't really get perfected until well into the 80s and acoupla generations of front-wheel-drive cars, so those old ones were noisy to begin with and VERY prone to failure, usually by their boots tearing up and letting in dirt and water. The trailing arm is like a big narrow triangle of stamped wrinkled sheet metal about a foot and a half long with a bushing at the point and a rear wheel bearing at the wide end, and the bearing ALWAYS fails, and the only way to get it is to buy the whole trailing arm. So, non-car-people who owned those, would notice a strange roaring sound coming from the left rear; they'd take it to "mechanic", who would diagnose it as, say, the left trailing arm; they'd pay "mechanic" out the wazoo to change it out. Then 2 weekends later, next time they drove the car, they'd hear another weird grumbling sound from the right rear, and go back to "mechanic" who would diagnose it as the right half-shaft, and they'd pay out the wazoo AGAIN for that. Then on the way home the rear brakes would stop working on one side and lock up on the other and the pedal would go to the floor, and "mechanic" would replace the calipers, butt since they were getting paid piecework, they wouldn't put in the ones you could get that had been rebuilt with Teflon seals and bored out to put in stainless steel sleeves so that they wouldn't fail in a few days, instead they'd go back with "genuine GM" which rusts again 3 months on down the road and turns to garbage EXACTLY like the ones they're replacing. Meanwhile, after another 3 weeks they'd hear a rumbling sound from the right rear, and lo! and behold, it's the RIGHT trailing arm this time. And on and on and on and on and on. Then about the time every one of those things has been replaced one at a time, the first one - the left trailing arm - fails again. So "mechanic" gets paid to start over on the whole cycle of "repairs", and he couldn't be blamed for the part failing because it came straight from GM anyway because GM was the sole source. "Mechanic" gets to put their kids through braces and college off of d00d's crappy Corvette crap that once they get their hook into the stuuuuupid owner, it's GUARANTEED cash flow for as long as moron owns the car. That's why their used to be SO MANY "mechanics" back in the day that worked on NOTHING but Vettes. It WASN'T because they were so good they could work on exotic expensive sports cars that nobody else knew how to fix. No... they had Chevy small or big block motors, Saginaw steering pumps, Turbo 350 or Muncie or T-10 4-speeds, and lots of other stuff that was EXACTLY the same as any other car on the road. It was because it was THE MOST PROFITABLE chassis on the road to work on, BY FAR, and once moron started using a particular "mechanic", "mechanic" knew they'd be back just about every weekend they tried to take the car out of the garage. Any given "mechanic" only needed to have about 6 or 8 regular clients, and they'd have enough work from just those few cars to make a damn good living, and if somebody that WASN'T a regular came to them, they'd hem and haw about "shop is full" and "I'll squeeze you in if I can butt I'll have to charge extra" and "might be a few weeks before I can get to it" and "gotta charge you for storage because it took so long for the parts to come in" and all the rest of that BS. People that owned them had enough money to buy them in the first place, and they were too ignorant or lazy or whatever to take care of them themselves. "Mechanic" would climb aboard the Vette gravy train and tell you to take your low-buck grocery cart to somebody else that had competition, and chisel THEM down on the price of repairs on your Family Truckster. I'm so snooty I ONLY work on Vettes.

Don't be THAT GUY that pays "mechanic" to indulge in their ANTIQUE car hobby FOR THEM. That's a 1-way ticket to BANKRUPTCY. Learn to participate in your hobby YOURSELF instead of paying "mechanic".

 

Thanks for the reply!! Appreciate your time. Tried in 2nd and still does it when hot after driving for 15 miles. When first drive car there is no noises