The iShares iBoxx $ Investment Grade Corporate Bond (NYSEArca: LQD) is second on the list of top-selling ETFs in 2012 with inflows of more than $7 billion as investors look beyond low-yielding Treasuries for any kind of steady income.

LQD has posted a total return of 11.4% year to date while iShares Barclays 20+ Year Treasury Bond (NYSEArca: TLT) has gained 6.1%, according to Morningstar.

The investment-grade corporate bond ETF holds $25.6 billion in assets and pays a 12-month yield of 3.8%, while the Treasury fund is yielding 2.7%, according to manager BlackRock (NYSE: BLK).

Yet some are worried that investment-grade bond prices have been pushed up so high and yields so low that investors buying into the fixed-income sector now could end up disappointed.

“Investors have been flocking to buy bonds issued by top-rated companies, putting them on pace for a record year of debt raising in the U.S. But some of the biggest fund managers warn that dangers are lurking in what were once seen as the safest investments,” The Wall Street Journal reports.

Some managers, including LQD manager BlackRock, say now could be one of the most dangerous times in decades to lend to investment-grade companies.

“Interest rates are so low and bond prices so high, they warn, that there is little room left for gains. Some worry that even a small increase in interest rates—a traditional enemy of bond returns—could eat away at bond prices,” the newspaper reports.