What kind of noise is it, and where is it coming from?
other...describe in detail?
Have you noticed any driveability issues since this noise started?
Also, how long ago did this start to happen?
I am basing my answer on what you told me. Since you mentioned that the noise sounds like a motorcycle muffler, then only thing that sounds like that is a muffler. I believe that your problem exists in the exhaust system. It may be possible that the exhaust manifold bolts got loose and that is why you are hearing the excessive noise.
If you have the transmission in park and you rev the engine, can you hear the noise?
From the recent work that was done to the heads. The exhaust system needs to be removed. So maybe the bolts got loose and thus causing the noise. It wouldnt be a bad idea to take the car in to the shop that did the work and have them check up on their work.
They shouldn't. I would ask them before they check anything.
You can drive around like that, no problem. I believe the problem is in the exhaust system. It may or may not be the bolts that got loose, it can be that there is a hole causing the noise and the bigger the hole the louder the noise. If the noise gradually started to get louder as time went by then that would indicate that you have an exhaust system that is deteriorating and needs to be replaced.
ps- I am posting as an info request, because you dont need to accept. Its on me. Have a good night and let me know what they tell you.
pps- if you for any reason you begin to smell the exhaust smoke, then it is dangerous to drive around like that. Since you didnt mention anything like that, I dont think its dangerous to drive. And I think that it may be that the exhaust system needs to be replaced(once again I based my answer on what you said).